* 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
Based on the feedback in
https://github.com/python/black/pull/845#issuecomment-490622711
- Remove TokenizerConfig, and add a field to Grammar instead.
- Pass the Grammar to the tokenizer.
- Rename `ASYNC_IS_RESERVED_KEYWORD` to `ASYNC_KEYWORDS` and
`ASYNC_IS_VALID_IDENTIFIER` to `ASYNC_IDENTIFIERS`.
