Features

--------

- Add support for end-to-end key backup (MSC1687) ([\#4019](https://github.com/matrix-org/synapse/issues/4019))

Bugfixes
--------

- Fix bug in event persistence logic which caused 'NoneType is not iterable' ([\#3995](https://github.com/matrix-org/synapse/issues/3995))
- Fix exception in background metrics collection ([\#3996](https://github.com/matrix-org/synapse/issues/3996))
- Fix exception handling in fetching remote profiles ([\#3997](https://github.com/matrix-org/synapse/issues/3997))
- Fix handling of rejected threepid invites ([\#3999](https://github.com/matrix-org/synapse/issues/3999))
- Workers now start on Python 3. ([\#4027](https://github.com/matrix-org/synapse/issues/4027))
- Synapse now starts on Python 3.7. ([\#4033](https://github.com/matrix-org/synapse/issues/4033))

Internal Changes
----------------

- Log exceptions in looping calls ([\#4008](https://github.com/matrix-org/synapse/issues/4008))
- Optimisation for serving federation requests ([\#4017](https://github.com/matrix-org/synapse/issues/4017))
- Add metric to count number of non-empty sync responses ([\#4022](https://github.com/matrix-org/synapse/issues/4022))
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