text/x-python3: add *.pyi glob (Python stub files)

Stub files are files containing type hints, they are written in normal Python 3 syntax, but generally leaving out runtime logic like variable initializers, function bodies, and default arguments.

https://www.python.org/dev/peps/pep-0484/#stub-files https://mypy.readthedocs.io/en/stable/stubs.html#stub-file-syntax