Special cases in iterable unpacking

[scoder]

These came up in CPython issue 34508. We should make sure that we support them as well, and follow whatever they decide over there.

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

    eval('*(1, 2), 3')

    a = []; a += *(1, 2), 3

    for i in *(1, 2), 3: pass

    def g(): yield *(1, 2), 3

1. write tests and find out if they work or not, see the extended_unpacking tests.
2. find the places in the parser (Parsing.py) where these are disallowed, see the hacker guide
3. fix the parsing issues, find out what else is not working for whatever reason.

Don't hesitate to submit an incomplete PR and ask for further guidance.