For some time I've noticed that my IDE is reporting mypy errors that the
pre-commit hook is not picking up. This is because [mypy
mirror](https://github.com/pre-commit/mirrors-mypy) runs in an isolated
pre-commit environment which does not have dependencies installed and it
enables `--ignore-missing-imports` to avoid errors.
This is [advised against by
mypy](https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-library-stubs-or-py-typed-marker)
for obvious reasons:
> We recommend avoiding --ignore-missing-imports if possible: it’s
equivalent to adding a # type: ignore to all unresolved imports in your
codebase.
This PR configures the mypy pre-commit hook to run in the virtual
environment and addresses the additional errors identified as a result.
It also introduces a minimal mypy config into the `pyproject.toml`
[mypy errors identified without the fixes in this
PR](https://github.com/user-attachments/files/15896693/mypyerrors.txt)
Also updates CI pypy versions to be 3.9 and 3.10 which are the currently
[supported
versions](https://www.pypy.org/posts/2024/01/pypy-v7315-release.html).
Otherwise latest cryptography doesn't ship with pypy3.8 wheels and is
unable to build on windows.
Also updates the `codecov-action` to v4 which fixed some intermittent
uploading errors.
