Blacken .py files in blib2to3 (#1011)

* Blacken .py files in blib2to3

This is in preparation for adding type annotations to blib2to3 in
order to compiling it with mypyc (#1009, which I can rebase on top of
this).

To enforce that it stays blackened, I just cargo-culted the existing
test code used for validating formatting. It feels pretty clunky now,
though, so I can abstract the common logic out into a helper if that
seems better. (But error messages might be less clear then?)

* Tidy up the tests