Add type annotations to smart_open

[mpenkov]

This is still a work in progress. I'm playing around with the idea to see if it's worth it.

[piskvorky]

Check out PEP 585: starting with python3.7, annotations become infinitely more readable.

May be worth the wait, until we drop py3.6?

[mpenkov]

Will that much change? From looking at that PEP, we'll go from using e.g. Dict[str, int] to dict[str, int] (and get rid of the from typing import (...).

[piskvorky]

To me – yes. I don't have anything against annotations but rarely use them, because I find they obfuscate more than clarify. Cons>pros.

A nicer, better integrated annotation syntax could tip the scales. Just a nice-to-have feature either way, nothing critical.