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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* [x] Pull request includes a [changelog
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* [x] [Code
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1 files changed, 1 insertions, 6 deletions

diff --git a/synapse/rest/admin/users.py b/synapse/rest/admin/users.py

index ad515bd5a3..076994b87e 100644
--- a/synapse/rest/admin/users.py
+++ b/synapse/rest/admin/users.py
@@ -27,7 +27,7 @@ from typing import TYPE_CHECKING, Dict, List, Optional, Tuple, Union
 
 import attr
 
-from synapse._pydantic_compat import HAS_PYDANTIC_V2
+from synapse._pydantic_compat import StrictBool
 from synapse.api.constants import Direction, UserTypes
 from synapse.api.errors import Codes, NotFoundError, SynapseError
 from synapse.http.servlet import (
@@ -56,11 +56,6 @@ from synapse.types.rest import RequestBodyModel
 if TYPE_CHECKING:
     from synapse.server import HomeServer
 
-if TYPE_CHECKING or HAS_PYDANTIC_V2:
-    from pydantic.v1 import StrictBool
-else:
-    from pydantic import StrictBool
-
 
 logger = logging.getLogger(__name__)