Don't error unnecessarily about new syntax in dependencies where we ignore errors anyways - #21883

I would be happy to explain more about the root cause, especially since I never actually made an issue particular to it, but essentially Trio has a lockfile (https://github.com/python-trio/trio/blob/main/test-requirements.txt) that conditionally has newer Sphinx versions that use new typing features. In particular, when I develop locally with my 3.14 Python, after a uv pip install <lockfile>, I get this:

a5rocks@whimsy ~/D/trio> uv run mypy
.venv/lib/python3.14/site-packages/sphinx/util/typing.py:29: error: Type statement is only supported in Python 3.12 and greater  [syntax]
Found 1 error in 1 file (errors prevented further checking)

and this has bothered me forever :-)

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

How does this work with the new parser? Or does this issue only affect the old parser?

Idk, I imagine it only affects old parser since I remember ruff's parser does some error recovery (and so hitting a syntax error won't block parsing the rest)

@A5rocks it looks like there is an epidemic of people not knowing what is going on in their PRs :-) Can you try the new parser? Also please test both with --native-parser and --no-native-parser.

On master the test case in this PR succeeds, though there's a bug (I think) in the test suite code that requires me to pass TEST_NATIVE_PARSER=True as an environment variable. I think it should be options.native_parser = options.native_parser or bool(os.environ.get("TEST_NATIVE_PARSER")) so tests can add a --native-parser flag to themselves?

So anyways, yes, it seems like only the old parser needs this PR.

OK, I am actually deleting this env var logic in my PR, so I can just merge this, and then the test will naturally switch to new parser when my PR is merged.