mypy stubs

[flammie]

I don't know if it's easy but using various ides or linters with python hfst gets warnings like so:

See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports
Cannot find implementation or library stub for module named 'hfst'
Cannot find implementation or library stub for module named 'libhfst'

if like swig can generate such stuff it would useful.