Enable Mypy strict mode - #9803

Given that this is considerably smaller than #9410, I suspect you're only fixing the strict mode problems that arose testing against Python 3.11.

#9410 had its sights set a bit higher - it actually wanted to correct strict mode problems for newer versions of Python as well, which fell into an unresolved debate about whether to test against those versions or not.

But that isn't necessarily something to stop this PR from being merged. I just suspect it doesn't exactly replace #9410.

Same result on all supported Pythons.

$ for py in 3.11 3.12 3.13 3.14; uvx -q --python=$py --with-requirements=.ci/requirements-mypy.txt mypy conftest.py selftest.py setup.py checks docs src winbuild Tests; end
Success: no issues found in 303 source files
Success: no issues found in 303 source files
Success: no issues found in 303 source files
Success: no issues found in 303 source files
$

radarhere@cfb3ab8 added to these changes gives failures on 3.12 - 3.14.

A review was requested, so I'll restate - the fundamental issue here isn't any implementation detail, it's the question of whether we should do this - #9414 (comment)

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?

python/mypy#12286 is an issue requesting the ability to check multiple versions at once. From the fact that it is still open, you might infer that this isn't an official priority.

#9410 (comment)

I don't think we have any "official" recommendations on this. But unofficially we recommend:

• Always run mypy on the oldest supported Python version.
• If your code has known platform-specific logic (e.g. Linux vs Windows), run the above on as many platforms as needed.
• You may also run mypy on the newest supported Python version (in this case it makes sense to use --enable-error-code=deprecated to see deprecations early).

I'm also not sure if we need to enable strict mode, some of our annotations are pretty complex already and I don't think the EpsImagePlugin.py and GifImagePlugin.py changes improve readability.

The ImageGrab.py fix is needed for macOS with the current config.