Support unpacking in yield/return

[JelleZijlstra]

Python 3.8 made a minor syntax change to allow

def g():
    rest = (4, 5, 6)
    return 1, 2, 3, *rest

and the equivalent with yield. See https://bugs.python.org/issue32117.

Black doesn't support this syntax yet.

[vemel]

This looks like a bug in plib2to3 as it cannot parse * character in this line.

Unfortunately, even in py38 plib2to3 return statement does not support startest: https://github.com/python/cpython/blob/3.8/Lib/lib2to3/Grammar.txt#L52

I can prepare a PR though

[vemel]

I prepared a PR above and added a test

[vemel]

Added PR to CPython as well. I hope black can switch to official lib2to3 soon.

[isidentical]

Looks like #1121 merged, guess we can close this?