literal return type for platform.system()

[Akuli]

Fixes python/mypy#8166 so that python/mypy#8461 is no longer needed.

[github-actions]

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

mypy (https://github.com/python/mypy.git)
+ mypyc/test/test_run.py:284: note: (Skipping most remaining errors due to unresolved imports or missing stubs; fix these first)

aiohttp (https://github.com/aio-libs/aiohttp.git)
+ aiohttp/helpers.py:218: error: Non-overlapping equality check (left operand type: "Literal['Linux']", right operand type: "Literal['Windows']")  [comparison-overlap]

werkzeug (https://github.com/pallets/werkzeug.git)
+ src/werkzeug/serving.py:70: error: Non-overlapping equality check (left operand type: "Literal['Linux']", right operand type: "Literal['Windows']")
- tests/test_wrappers.py:710: error: Function is missing a return type annotation

[Akuli]

Now that I see the mypy_primer output, I'm no longer sure about whether this is a good idea, because it now warns about Literal["Linux"] == Literal["Windows"].

[JelleZijlstra]

Yes, that's unfortunate. Maybe that mypy option can be taught to not error when comparing two literals that aren't part of a union.

[srittau]

This is an interesting case. Looked at from one side, this change is correct. Looked at from another side it isn't. I would suggest for now not to use conditionals and just return Literal['Linux', 'Windows', 'Darwin'] | str until mypy is changed. After that, I think this PR is worth a try.

[JelleZijlstra]

Going to close this as stale for now to keep the list of open PRs manageable. If you're still interested in seeing this through, feel free to reopen or open a new PR.