Add support for pydantic v2 via pydantic.v1 compat module

[gotmax23]

Add support for pydantic v2 via pydantic.v1 compat module

This adds support for pydantic v2 by using the pydantic.v1 compat module when pydantic v2 is installed and otherwise falling back to the pydantic module.

In order to satisfy mypy, we only import pydantic.v1 during type checking. This allows proper type checking without needing to clutter the code with type: ignores.

Once some time is passed, we can entirely drop compatibility for pydantic v1 and start using the actual pydantic v2 code to reap the additional performance benefits and other improvements of pydantic v2.

Relates: https://github.com/matrix-org/synapse/issues/15858

(I didn't use Fixes:, as we should track enabling full support for pydantic v2)

Pull Request Checklist

• [x] Pull request is based on the develop branch
• [x] Pull request includes a changelog file. The entry should:
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• [x] Pull request includes a sign off
• [x] Code style is correct (run the linters)

[clokep]

See also #16331.

[gotmax23]

@clokep, do you mind approving the CI run?

[clokep]

It looks like one of the CI jobs is failing too: https://github.com/matrix-org/synapse/actions/runs/6230975624/job/16994483499#step:10:32

[gotmax23]

It looks like one of the CI jobs is failing too: https://github.com/matrix-org/synapse/actions/runs/6230975624/job/16994483499#step:10:32

Those tests are running with packaging==16.1 from 2016 :fearful:. Can this be updated to at least version 20 from 2019 that has https://github.com/pypa/packaging/commit/f1b2b2222c8832a5a01f328afb46c9f898e67a1e?

[gotmax23]

I pushed to address the merge conflicts and feedback. @clokep, can you please approve CI again :slightly_smiling_face:?

[gotmax23]

It looks like one of the CI jobs is failing too: https://github.com/matrix-org/synapse/actions/runs/6230975624/job/16994483499#step:10:32

Those tests are running with packaging==16.1 from 2016 😨. Can this be updated to at least version 20 from 2019 that has pypa/packaging@f1b2b22?

I bumped the minimum packaging version.

[6543]

Alpine packager: ACK

[arkamar]

Gentoo: ACK, tested with pydantic 1.10.12 and 2.3.0.

[clokep]

Just need CI to churn and this is good to go. 👍

[gotmax23]

Thanks @clokep!