bpo-34508: allow unparenthesized star-unpacking expressions in return statements

[mdickinson]

This PR makes the following (currently illegal) syntax legal:

def f():
    return *(1, 2), *(3, 4)

This eliminates an inconsistency between the target of a return and the target of a simple assignment: the following syntax is already legal:

def f():
    x = *(1, 2), *(3, 4)
    return x

https://bugs.python.org/issue34508

[mdickinson]

Closing. I accidentally created a branch on python/cpython instead of in my own fork. Apologies.

EDIT: Recreated as #8942.