Make the package PEP 561 Compatible

[ashwoods]

You can make the package PEP 561 compatible by adding a py.typed file. I'd be happy to make a PR if desired.
Thx!

https://mypy.readthedocs.io/en/latest/installed_packages.html#installed-packages

[pwwang]

Yes, please go ahead.
But I haven't tested the whole package with mypy yet. You may also add it to CI if you want.