Create PEP 561 compliant library stub for type checking

[Anomalocaridid]

Currently, i3ipc-python does not work with Python type checkers like mypy. In order to support type checking, it needs a library stub as specified in PEP 561.

This isn't a critical issue, but it would certainly be much appreciated.

[acrisci]

I would accept this into the library.

[palfrey]

#205 does this.

[eternal-sorrow]

While the PR stays unmerged for some reason, I temporarily did this in my dev environment: pip install git+https://github.com/palfrey/i3ipc-python@add-typing.