Use blib2to3 parser from black package to support match statement

[nrser]

In response to issue #79.

The built-in lib2to3 does not support pattern matching (Python 3.10+):

https://docs.python.org/3.11/library/2to3.html#module-lib2to3

The black project managed to get some level of parsing support for
match out of their modified version blib2to3, see:

1. Support Python 3.10 pattern matching syntax psf/black#2242
2. black/parser: partial support for pattern matching psf/black#2586

This change adds black as a dependency and switches to using
blib2to3 to parse.

Tests pass, and I am currently using it on an actual project of moderate complexity, but this is a significant change to core functionality... caution is advised. It seems like blib2to3 itself is a bit of hack/stop-gap, and that's not counting my hasty integration with docspec-python.

However, for users now in a broken state due to match syntax in their source files this may be worth a shot.

[NiklasRosenstein]

Thanks a lot for this @nrser 🙌 I hope I can take a closer look later this week.

blib2to3 does indeed sound like a stop gap solution, but I'm happy to use it as long as it unblocks using docspec with the new syntax. I was hoping to migrating to libCST in the future (#3) but that's a big chunk of work.

[NiklasRosenstein]

The update-changelogs task action isn't really working for PRs from forks I guess (NiklasRosenstein/slap#90), so I'm ignoring that failed test.

Thanks a lot for this @nrser!