Support compilation with mypyc (#1009)

* Make most of blib2to3 directly typed and mypyc-compatible

This used a combination of retype and pytype's merge-pyi to do the
initial merges of the stubs, which then required manual tweaking to
make actually typecheck and work with mypyc.

Co-authored-by: Sanjit Kalapatapu <sanjitkal@gmail.com>
Co-authored-by: Michael J. Sullivan <sully@msully.net>

* Make black able to compile and run with mypyc

The changes made fall into a couple categories:
 * Fixing actual type mistakes that slip through the cracks
 * Working around a couple mypy bugs (the most annoying of which being
   that we need to add type annotations in a number of places where
   variables are initialized to None)

Co-authored-by: Sanjit Kalapatapu <sanjitkal@gmail.com>
Co-authored-by: Michael J. Sullivan <sully@msully.net>