ty: un-ignore invalid-type-form

[Khushi-Tyagi9]

Part of #15187.

Image here is the PIL.Image module, not the class, so ty flagged it under
invalid-type-form. Changed the annotations to Image.Image in
mean_threshold, otsu_threshold, change_brightness and change_contrast, and
removed the rule from the ignore list in pyproject.toml.

Ran ty with the rule enabled and it's clean now. Only annotations changed,
nothing at runtime.

Describe your change

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Add or change doctests? -- Note: Please avoid changing both code and tests in a single pull request.
• Documentation change?

Checklist

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new Python files are placed inside an existing directory.
• All filenames are in all lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming conventions.
• All function parameters and return values are annotated with Python type hints.
• All functions have doctests that pass the automated testing.
• All new algorithms include at least one URL that points to Wikipedia or another similar explanation.
• If this pull request resolves one or more open issues, then the description above includes the issue number(s) with a closing keyword: "Fixes #ISSUE-NUMBER".

[cclauss]

@priya-sundaram-dev, please review.

[priya-sundaram-dev]

LGTM — the four annotations that changed are genuinely the correct fix: Image is the module (from PIL import Image), and the actual image type is the Image.Image class, so -> Image.Image is right and -> Image was the invalid-type-form. With those fixed, dropping rules.invalid-type-form = "ignore" is safe and the ty CI check is green on this head, so the rule is now genuinely enforced rather than suppressed.

One small consistency note (non-blocking, fine as a follow-up): the parameter annotations still read image: Image / img: Image, i.e. the same module-vs-class distinction on the input side. ty isn't flagging them today so CI stays green, but tightening those to Image.Image too would make each signature read consistently. Happy either way — the PR as-is achieves its stated goal of un-ignoring the rule with green CI.

[priya-sundaram-dev]

Came to this just after the merge — for the record it looks right to me. Since these files do from PIL import Image, the bare Image annotation was actually the module, so ty was correct to flag invalid-type-form; Image.Image is the intended PIL.Image.Image class. Pure annotation change, no runtime effect, and dropping the rule from the ignore list is the right cleanup. Nice one, @Khushi-Tyagi9. 👍