Run Lint against multiple Python versions

[radarhere]

While exploring #9410, the user found that our type hints are not being adequately checked across different Python versions - jorenham#1 (comment)

This PR stops specifying the mypy Python version in pyproject.toml, so that it defaults to the current version being run, and adds a matrix to our Lint job to check multiple versions.

Python 3.10 required tomli - https://github.com/python-pillow/Pillow/actions/runs/21516054322/job/61994586354
and Python >= 3.12 required some changes in Buffer handling - https://github.com/python-pillow/Pillow/actions/runs/21516054322/job/61994586341

[hugovk]

Do we really need to run mypy on every version? Should we run on the oldest supported, or latest? Is there authoritative guidance on best practices here?

If we do decide to type check on multiple versions, let's move mypy to its own job (can name it like typing for when we switch to ty or pyrefly or something) and only apply the matrix there. The other linting can remain on an single version.

And can we continue to use a new Python interpreter but call mypy with --python-version instead of setting up older interpreters?

https://mypy.readthedocs.io/en/stable/command_line.html#cmdoption-mypy-python-version