pywin32: Use PEP 570 positional-only argument syntax and fix arguments

[Avasam]

The pywin32 part of #11237

Split into multiple commits for ease of review:

1. The first commit assumes all c-extension modules are positional-only
2. The second commit removes redundant __ prefixes
3. For the third commit I searched the source code for all instances of PyArg_ParseTupleAndKeywords and updated all methods in the stubs that should support kwargs, and tested which ones accepted default arguments (except for methods in like 2-3 credential-related classes in _win32typing.pyi that I could figure out how to instantiate). This includes a bunch of fixes to argument order and return types I noticed along the way.

[Avasam]

That's an interesting stubtest error (at the very least the "remove leading double underscore" message should be updated):

error: win32.lib.pywintypes.error.__init__ is inconsistent, stub argument "self" should be positional or keyword (remove leading double underscore)
Stub: in file E:\Users\Avasam\Documents\Git\typeshed\stubs\pywin32\win32\lib\pywintypes.pyi:14
def (win32.lib.pywintypes.error, builtins.int, builtins.str, builtins.str)
Runtime: in file <string>
def (self, *args, **kw)

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉