As noticed in https://github.com/python-trio/trio/pull/3162#issuecomment-2557061317 & https://github.com/python-trio/trio/pull/3162#issuecomment-2557465927 we have code guarded by `if sys.version_info > (3, ...)` that currently isn't type checked at all. So we should probably test both lowest & highest supported python version with mypy. Should be fairly straightforward: duplicate the code in `check.sh` and fix the one error in `_path.py`.