[CI] Fix mypy linting

[antonymilne]

Description

#397 bumped our pre-commit mypy from 1.8.0 to 1.9.0, which seemed to flag new errors that were still there even after reversion to 0.18.0 in #403.

The reason for these new errors was not actually the change to mypy version but that there was a new pydantic release at about the same time. Pinning the pydantic version used by mypy fixes this.

There is no caching in our CI. There is caching on pre-commit.ci but it works well and will always be running the correct version specified in our .pre-commit-config.yaml. So both our CI and pre-commit.ci are still perfect sources of truth and working correctly. Behaviour locally should be the identical to these, but just in case it's not then pre-commit clean should fix it by clearing your local cache.

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[antonymilne]

FYI @maxschulz-COL @stichbury this should have been on the skip list for https://pre-commit.ci/ already, because vale just flags errors rather than fixing them. The only things we run on pre-commit.ci are things that can fix stuff.

[huong-li-nguyen]

Ahh, amazing!! 🚀