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

[A5rocks]

This fixes #21178 (at least my own root cause for it). This is actually a bit trickier than I thought (you can read that issue to see what I thought would work, which was naive!). However, even though we use the ast module and cannot make the errors nonblocking, we can work around the root cause of that issue i.e. that new features in dependencies can cause mypy to fail.

[A5rocks]

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 :-)

[github-actions]

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