Adds py.typed and fixes mypy issues

[palfrey]

i3ipc now has no complaints from mypy. This fixes #187.

My current test setup is:

0. Do existing i3ipc-python setup
1. pip install mypy types-python-xlib
2. Go into the test folder.
3. Repeatedly run (cd .. && pip install . && python -m mypy . --exclude build --exclude examples --exclude docs --exclude setup.py) as I made changes until it stopped breaking.

jazzband/dj-database-url@7ee1eea demos this sort of check in CI environment, but can't do that here yet because of #204.

[eternal-sorrow]

Any progress with this?

[eternal-sorrow]

Okay, I tested this and it has a lot of problems with aio types specifically, where it mistypes i3ipc.aio.X types into i3ipc.X types, because i3ipc.aio classes are subclasses of i3ipc classes. Also, classes are missing many of their attributes because those attributes are assigned dynamically and mypy cannot infer their presence.

[eternal-sorrow]

aio.Connection.on() handler argument should be Optional[Callable[['Connection', IpcBaseEvent], Union[Coroutine, None]]] because it supports both coroutines and normal functions as event handlers. The same for .off().

[eternal-sorrow]

I created a new PR with full annotations: #214