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
<!-- Please read
https://element-hq.github.io/synapse/latest/development/contributing_guide.html
before submitting your pull request -->
* [x] Pull request is based on the develop branch
* [x] Pull request includes a [changelog
file](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#changelog).
The entry should:
- Be a short description of your change which makes sense to users.
"Fixed a bug that prevented receiving messages from other servers."
instead of "Moved X method from `EventStore` to `EventWorkerStore`.".
- Use markdown where necessary, mostly for `code blocks`.
- End with either a period (.) or an exclamation mark (!).
- Start with a capital letter.
- Feel free to credit yourself, by adding a sentence "Contributed by
@github_username." or "Contributed by [Your Name]." to the end of the
entry.
* [x] [Code
style](https://element-hq.github.io/synapse/latest/code_style.html) is
correct
(run the
[linters](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#run-the-linters))
During the migration the automated script to update the copyright
headers accidentally got rid of some of the existing copyright lines.
Reinstate them.
