Add support for type checking (PEP 561) #39
Description
Currently running mypy checks against code, that is using celpy, results in the following output:
error: Skipping analyzing "celpy": module is installed, but missing library stubs or py.typed marker [import]
note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
error: Type of variable becomes "Any" due to an unfollowed import [no-any-unimported]
This can be fixed by adding py.typed file (https://peps.python.org/pep-0561/#packaging-type-information). I've tried adding it locally to the celpy package and mypy checks have passed.