I found that the parser fails to handle correctly the (incorrect) case
where the single-star (*), used for delimiting keyword-only arguments,
is immediately followed by a **keywords parameter:

>>> def f(*, **kw):
...   pass
... 
python: Python/ast.c:652: handle_keywordonly_args: Assertion `kwonlyargs
!= ((void *)0)' failed.
[1]    7872 abort (core dumped)  ./python