Dropping support for Python 3.7

[Avasam]

Python 3.7 has reached EOL June 27 2023.

As this PR shows, dropping Python 3.7 doesn't bring as much as dropping 3.6 did tooling-wise, and 3.8 will syntax wise. But it brings a nice improvement to dealing with DLL location and PATH issues.

• os.add_dll_directory can now be used unconditionally. No more PATH environment variable shenanigans!
• The last version of https://pypi.org/project/pypiwin32/ is 223. pywin32==223 supported up to Python 3.7 at most. Leaving pypiwin32 behind for good.
• Allows annotating global variables: Annotate ambiguous global variables #2323

Also of interest:

• New positional-only syntax. (which is the reason mypy 1.5 decided to drop 3.7). But pywin32 doesn't even use the old syntax (def foo(__bar):...)
• Self-documenting f-strings. I wouldn't do that in this PR, and I'd want to keep reducing the mix of f-string, printf-style and format calls first.
• https://docs.python.org/3/whatsnew/3.8.html#pep-587-python-initialization-configuration (I'm not a C-Python programmer, no idea if any of these would be useful for pywin32)
• https://docs.python.org/3/whatsnew/3.8.html#debug-build-uses-the-same-abi-as-release-build (would this affect pywin32's considerations for debug builds?)

I understand if you'd like to have one more release (307) supporting Python 3.7 before dropping it.

[mhammond]

Conceptually this is fine, but if there's nothing which requires this it would be ideal if I could make another final release supporting 2.7 given it went EOL after the most recent pywin32 release.

[Avasam]

Setting as draft until the next release.
Speaking of which, I should build up a changelog of all the recent cleanup and fixes.