Add py.typed and annotate most of the classes and methods

[eternal-sorrow]

Supersedes #205, fixes #187.

I annotated types in most of the classes and methods instead of keeping it unannotated and just plugging mypy complaints with # type: ignore. With some notable exceptions:

• I didn't touch synchronizer, because I don't understand how it works, looks like it has some undefined variables.
• I didin't use any advanced trickery with types to make aio and non-aio classes compatible. I had to use # type: ignore here and there because of this, this could be avoided using some base class for Connection and aio.Connection, and also come common base class for Con and aio.Con instead of just inheriting one from the other.
• I used some TypeVars for Con and replies. I tried to keep all of this compatible with python 3.9, so I didn't use any recent additions to typing system. This also caused some # type: ignores because advanced parameter annotations are not available. In my opinion, callbacks with no event argument should be deprecated though.

I also used a contravariant TypeVar to annotate the IpcBaseEvent argument for event handlers. This makes it possible for the users of this library to annotate their callbacks with actual event types they are expecting to receive (WindowEvent for example).