Make package PEP 561 compliant

[Gunni]

When running mypy on a script that uses this package, I get:

error: Skipping analyzing "pystemd.systemd1": 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

Could this module be made PEP 561 compliant?