Parse Python 3.9+ parenthesized context managers

Since the upstream grammar for this is not LR(1), we abuse LALRPOP
macros and the Into/TryInto traits to build a cover grammar that
converts to either tuples or `with` items after additional validation.
It’s annoying and ugly, but something like this is basically our only
option short of switching to a more powerful parser algorithm.

Fixes #4145.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

Author: andersk