add mypy run on py3.13

[jakkdl]

As noticed in #3162 (comment) & #3162 (comment) 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.