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
1 job for python-stubs in 2 minutes and 16 seconds (queued for 1 second)