import pydantic objects from the `_pydantic_compat` module (#17667)

This PR changes `from pydantic import BaseModel` to `from
synapse._pydantic_compat import BaseModel` (as well as `constr`,
`conbytes`, `conint`, `confloat`).

It allows `check_pydantic_models.py` to mock those pydantic objects only
in the synapse module, and not interfere with pydantic objects in
external dependencies.

This should solve the CI problems for #17144, which breaks because
`check_pydantic_models.py` patches pydantic models from
[scim2-models](https://scim2-models.readthedocs.io/).

/cc @DMRobertson @gotmax23
fixes #17659 


### Pull Request Checklist

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1 files changed, 1 insertions, 6 deletions

diff --git a/synapse/storage/background_updates.py b/synapse/storage/background_updates.py

index efe4238036..1194b58ffb 100644
--- a/synapse/storage/background_updates.py
+++ b/synapse/storage/background_updates.py
@@ -40,7 +40,7 @@ from typing import (
 
 import attr
 
-from synapse._pydantic_compat import HAS_PYDANTIC_V2
+from synapse._pydantic_compat import BaseModel
 from synapse.metrics.background_process_metrics import run_as_background_process
 from synapse.storage.engines import PostgresEngine
 from synapse.storage.types import Connection, Cursor
@@ -49,11 +49,6 @@ from synapse.util import Clock, json_encoder
 
 from . import engines
 
-if TYPE_CHECKING or HAS_PYDANTIC_V2:
-    from pydantic.v1 import BaseModel
-else:
-    from pydantic import BaseModel
-
 if TYPE_CHECKING:
     from synapse.server import HomeServer
     from synapse.storage.database import (