bpo-33064: lib2to3: support trailing comma after *args and **kwargs (#6096)

New tests also added.

I also made the comments in line with the builtin Grammar/Grammar. PEP 306 was
withdrawn, Kees Blom's railroad program has been lost to the sands of time for
at least 16 years now (I found a python-dev post from people looking for it).
3 files changed