[flake8-pyi] Allow simple assignments to None in enum class scopes (PYI026)

[AlexWaygood]

Fixes #11117

[AlexWaygood]

I also deduplicated the two is_enum functions between flake8-slots and flake8-pyi, since the flake8-pyi one was way more complex than it needed to be, and was also missing an enum class that the flake8-slots function included.

[charliermarsh]

Thanks.

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

ℹ️ ecosystem check detected linter changes. (+0 -1 violations, +0 -0 fixes in 1 projects; 43 projects unchanged)

python/typeshed (+0 -1 violations, +0 -0 fixes)

ruff check --no-cache --exit-zero --ignore RUF9 --output-format concise --preview --select E,F,FA,I,PYI,RUF,UP,W

- stdlib/uuid.pyi:12:5: PYI042 Type alias `unknown` should be CamelCase

Changes by rule (1 rules affected)

code	total	+ violation	- violation	+ fix	- fix
PYI042	1	0	1	0	0

[AlexWaygood]

- [stdlib/uuid.pyi:12:5:](https://github.com/python/typeshed/blob/d0f2be92aba2e007841eec073cb47d7f86722792/stdlib/uuid.pyi#L12) PYI042 Type alias `unknown` should be CamelCase

hmmm, seems like a good change but not the one I expected lol

[AlexWaygood]

hmmm, seems like a good change but not the one I expected lol

Well, I confirmed locally that this fixes the false positive I was trying to fix, so I'll just add another test for the other fixed bug that the ecosystem check highlighted

[AlexWaygood]

And, I can't repro the change the ecosystem is reporting locally either. Weird. Anyway, I'm pretty confident in this change, so I'll merge...