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The problem is in Python/ast.c around line 666.  See the
comment:

/* def foo((x)): setup for checking NAME below. */

The code is not sufficient, we need a loop and need to
handle various combinations of:

def f(((((x))),)),))): pass

I don't know if the parens above match, but the general idea
is that there could be a bunch of parens and commas at
various places.  I'm not sure how the above should be
interpreted.