Introduce Y058: Use (Async)Iterator, not (Async)Generator, for simple __(a)iter__ methods

[AlexWaygood]

Closes #423

[github-actions]

⚠ Flake8 diff showing the effect of this PR on typeshed:

> ./stubs/fpdf2/fpdf/structure_tree.pyi:48:5: Y058 Use "Iterator" as the return value for simple "__iter__" methods, e.g. "def __iter__(self) -> Iterator[Incomplete]: ..."
> ./stubs/setuptools/pkg_resources/__init__.pyi:35:5: Y058 Use "Iterator" as the return value for simple "__iter__" methods, e.g. "def __iter__(self) -> Iterator[Distribution]: ..."
> ./stubs/setuptools/pkg_resources/__init__.pyi:64:5: Y058 Use "Iterator" as the return value for simple "__iter__" methods, e.g. "def __iter__(self) -> Iterator[str]: ..."

[AlexWaygood]

Those hits all look good to me.

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https://github.com/python/typeshed/blob/d14c5f82c881af92bd52519193f2b64aa4403999/stubs/fpdf2/fpdf/structure_tree.pyi#L48

fpdf2 runtime: https://github.com/py-pdf/fpdf2/blob/a4d05f5f5cdfd6451441f9465e1ae65d438836f8/fpdf/structure_tree.py#L145-L150

https://github.com/python/typeshed/blob/d14c5f82c881af92bd52519193f2b64aa4403999/stubs/setuptools/pkg_resources/__init__.pyi#L35

https://github.com/python/typeshed/blob/d14c5f82c881af92bd52519193f2b64aa4403999/stubs/setuptools/pkg_resources/__init__.pyi#L64

pkg_resources runtime: https://github.com/pypa/setuptools/blob/main/pkg_resources/__init__.py