DOCS: recommend poetry where appropriate

In particular the documentaion for contributors should only advocate
poetry.

I am not thrilled at the propsect of now having N+1 installation
methods---but at the very least we ought to mention poetry here.

4 files changed, 51 insertions, 56 deletions

diff --git a/README.rst b/README.rst
index 595fb5ff62..0b30207afc 100644
--- a/README.rst
+++ b/README.rst
@@ -293,24 +293,26 @@ directory of your choice::
     git clone https://github.com/matrix-org/synapse.git
     cd synapse
 
-Synapse has a number of external dependencies, that are easiest
-to install using pip and a virtualenv::
+Synapse has a number of external dependencies. We maintain a fixed development
+environment using [poetry](). First, install poetry. We recommend
 
-    python3 -m venv ./env
-    source ./env/bin/activate
-    pip install -e ".[all,dev]"
+    pip install --user pipx
+    pipx install poetry
 
-This will run a process of downloading and installing all the needed
-dependencies into a virtual env. If any dependencies fail to install,
-try installing the failing modules individually::
+but see the `poetry installation docs <https://python-poetry.org/docs/#installation>`_
+for more details. Then ask poetry to create a virtualenvironment from the project
+and install Synapse's dependencies::
+
+    poetry install --extras "all test"
 
-    pip install -e "module-name"
+This will run a process of downloading and installing all the needed
+dependencies into a virtual env.
 
 We recommend using the demo which starts 3 federated instances running on ports `8080` - `8082`
 
-    ./demo/start.sh
+    poetry run ./demo/start.sh
 
-(to stop, you can use `./demo/stop.sh`)
+(to stop, you can use `poetry run ./demo/stop.sh`)
 
 See the [demo documentation](https://matrix-org.github.io/synapse/develop/development/demo.html)
 for more information.
@@ -318,14 +320,14 @@ for more information.
 If you just want to start a single instance of the app and run it directly::
 
     # Create the homeserver.yaml config once
-    python -m synapse.app.homeserver \
+    poetry run python -m synapse.app.homeserver \
       --server-name my.domain.name \
       --config-path homeserver.yaml \
       --generate-config \
       --report-stats=[yes|no]
 
     # Start the app
-    python -m synapse.app.homeserver --config-path homeserver.yaml
+    poetry run python -m synapse.app.homeserver --config-path homeserver.yaml
 
 
 Running the unit tests
@@ -334,7 +336,7 @@ Running the unit tests
 After getting up and running, you may wish to run Synapse's unit tests to
 check that everything is installed correctly::
 
-    trial tests
+    poetry run trial tests
 
 This should end with a 'PASSED' result (note that exact numbers will
 differ)::
diff --git a/docs/code_style.md b/docs/code_style.md
index ebda6dcc85..10b5bb6a7b 100644
--- a/docs/code_style.md
+++ b/docs/code_style.md
@@ -6,13 +6,8 @@ The Synapse codebase uses a number of code formatting tools in order to
 quickly and automatically check for formatting (and sometimes logical)
 errors in code.
 
-The necessary tools are detailed below.
-
-First install them with:
-
-```sh
-pip install -e ".[lint,mypy]"
-```
+The necessary tools are detailed below. They should be installed by poetry as part
+of Synapse's dev dependencies. See [the contributing guide](https://matrix-org.github.io/synapse/develop/development/contributing_guide.html#4-install-the-dependencies) for instructions.
 
 -   **black**
 
diff --git a/docs/development/contributing_guide.md b/docs/development/contributing_guide.md
index 071202e196..639b3d8d14 100644
--- a/docs/development/contributing_guide.md
+++ b/docs/development/contributing_guide.md
@@ -48,16 +48,16 @@ can find many good git tutorials on the web.
 
 # 4. Install the dependencies
 
-Once you have installed Python 3 and added the source, please open a terminal and
-setup a *virtualenv*, as follows:
+Synapse uses the [poetry](https://python-poetry.org/) project to lock its dependencies
+and development environment. Once you have installed Python 3 and added the
+source, you should [install poetry](https://python-poetry.org/docs/#installation).
+We recommend installing `poetry` using `pipx`; see their instructions for details.
+
+Next, open a terminal and install dependencies as follows:
 
 ```sh
 cd path/where/you/have/cloned/the/repository
-python3 -m venv ./env
-source ./env/bin/activate
-pip install wheel
-pip install -e ".[all,dev]"
-pip install tox
+poetry install --extras "all test"
 ```
 
 This will install the developer dependencies for the project.
@@ -117,11 +117,10 @@ The linters look at your code and do two things:
 - ensure that your code follows the coding style adopted by the project;
 - catch a number of errors in your code.
 
-The linter takes no time at all to run as soon as you've [downloaded the dependencies into your python virtual environment](#4-install-the-dependencies).
+The linter takes no time at all to run as soon as you've [downloaded the dependencies](#4-install-the-dependencies).
 
 ```sh
-source ./env/bin/activate
-./scripts-dev/lint.sh
+poetry run ./scripts-dev/lint.sh
 ```
 
 Note that this script *will modify your files* to fix styling errors.
@@ -131,15 +130,13 @@ If you wish to restrict the linters to only the files changed since the last com
 (much faster!), you can instead run:
 
 ```sh
-source ./env/bin/activate
-./scripts-dev/lint.sh -d
+poetry run ./scripts-dev/lint.sh -d
 ```
 
 Or if you know exactly which files you wish to lint, you can instead run:
 
 ```sh
-source ./env/bin/activate
-./scripts-dev/lint.sh path/to/file1.py path/to/file2.py path/to/folder
+poetry run ./scripts-dev/lint.sh path/to/file1.py path/to/file2.py path/to/folder
 ```
 
 ## Run the unit tests (Twisted trial).
@@ -148,16 +145,14 @@ The unit tests run parts of Synapse, including your changes, to see if anything
 was broken. They are slower than the linters but will typically catch more errors.
 
 ```sh
-source ./env/bin/activate
-trial tests
+poetry run trial tests
 ```
 
 If you wish to only run *some* unit tests, you may specify
 another module instead of `tests` - or a test class or a method:
 
 ```sh
-source ./env/bin/activate
-trial tests.rest.admin.test_room tests.handlers.test_admin.ExfiltrateData.test_invite
+poetry run trial tests.rest.admin.test_room tests.handlers.test_admin.ExfiltrateData.test_invite
 ```
 
 If your tests fail, you may wish to look at the logs (the default log level is `ERROR`):
@@ -169,7 +164,7 @@ less _trial_temp/test.log
 To increase the log level for the tests, set `SYNAPSE_TEST_LOG_LEVEL`:
 
 ```sh
-SYNAPSE_TEST_LOG_LEVEL=DEBUG trial tests
+SYNAPSE_TEST_LOG_LEVEL=DEBUG poetry run trial tests
 ```
 
 By default, tests will use an in-memory SQLite database for test data. For additional
@@ -180,7 +175,7 @@ database state to be stored in a file named `test.db` under the trial process'
 working directory. Typically, this ends up being `_trial_temp/test.db`. For example:
 
 ```sh
-SYNAPSE_TEST_PERSIST_SQLITE_DB=1 trial tests
+SYNAPSE_TEST_PERSIST_SQLITE_DB=1 poetry run trial tests
 ```
 
 The database file can then be inspected with:
diff --git a/docs/upgrade.md b/docs/upgrade.md
index 062e823333..8f52fdda7b 100644
--- a/docs/upgrade.md
+++ b/docs/upgrade.md
@@ -19,32 +19,35 @@ this document.
     packages](setup/installation.md#prebuilt-packages), you will need to follow the
     normal process for upgrading those packages.
 
+-   If Synapse was installed using pip then upgrade to the latest
+    version by running:
+
+    ```bash
+    pip install --upgrade matrix-synapse
+    ```
+
 -   If Synapse was installed from source, then:
 
-    1.  Activate the virtualenv before upgrading. For example, if
-        Synapse is installed in a virtualenv in `~/synapse/env` then
+    1.  Obtain the latest version of the source code. Git users can run
+        `git pull` to do this.
+
+    2.  If you're running Synapse in a virtualenv, make sure to activate it before
+        upgrading. For example, if Synapse is installed in a virtualenv in `~/synapse/env` then
         run:
 
         ```bash
         source ~/synapse/env/bin/activate
+        pip install --upgrade .
         ```
+        Include any relevant extras with `-e`, e.g. `pip install --upgrade .[postgres,oidc]`.
 
-    2.  If Synapse was installed using pip then upgrade to the latest
-        version by running:
-
-        ```bash
-        pip install --upgrade matrix-synapse
-        ```
-
-        If Synapse was installed using git then upgrade to the latest
-        version by running:
-
+    3.  If you're using `poetry` to manage a Synapse installation, run:
         ```bash
-        git pull
-        pip install --upgrade .
+        poetry install
         ```
+        Include any relevant extras with `-E`, e.g. `poetry install -E "postgres oidc"`.
 
-    3.  Restart Synapse:
+    4.  Restart Synapse:
 
         ```bash
         synctl restart