Type setuptools compiler attributes

[Avasam]

The compiler attr in commands is weird, it's never set in finalize_options, but rather in run instead. (normally we treat anything in finalize_options in setuptools the same as if it's been initialized in __init__)
With downstream command subclasses also intended to override finalize_options, this means compiler may not be an actual Compiler class instance yet.

Tentatively opening to see ecosystem check.

This would, once again, benefit from AnyOf: python/typing#566

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

setuptools (https://github.com/pypa/setuptools)
+ setuptools/command/build_ext.py:226: error: Argument "compiler" to "new_compiler" has incompatible type "Compiler | str | None"; expected "str | None"  [arg-type]
+ setuptools/command/build_clib.py:77: error: Item "str" of "Compiler | str | None" has no attribute "object_filenames"  [union-attr]
+ setuptools/command/build_clib.py:77: error: Item "None" of "Compiler | str | None" has no attribute "object_filenames"  [union-attr]
+ setuptools/command/build_clib.py:89: error: Item "str" of "Compiler | str | None" has no attribute "compile"  [union-attr]
+ setuptools/command/build_clib.py:89: error: Item "None" of "Compiler | str | None" has no attribute "compile"  [union-attr]
+ setuptools/command/build_clib.py:101: error: Item "str" of "Compiler | str | None" has no attribute "create_static_lib"  [union-attr]
+ setuptools/command/build_clib.py:101: error: Item "None" of "Compiler | str | None" has no attribute "create_static_lib"  [union-attr]

[Avasam]

So this is an issue for setuptools itself... I might be able to push for upstream changes, but I'm not yet sure how to do so in a backwards compatible way.

Maybe I'll have to keep these as Any