Explicitly export types from __init__.py

[dbarnett]

Adds explicit import aliases per
https://mypy.readthedocs.io/en/stable/command_line.html#cmdoption-mypy-no-implicit-reexport.

Fixes #53.

[dbarnett]

Please LMK if you prefer to use the __all__ approach instead, and any other important symbols I should also export while I'm in here.

[htgoebel]

This adds semantic understood by mypy only. I've never seen this and without your link to teh mypy documentation I'd say, this is bad code.

__all__ is much more commonly used and much more understood. I suggest using that.