Here is a patch. It also includes tests that would have detected this bug. It also corrects a case when getcallargs raised an exception with a different message (there are tests also for this):

>>> def f(**kwargs): pass
... 
>>> f(1, a=2)
Traceback (most recent call last):
    ...
TypeError: f() takes exactly 0 positional arguments (2 given)
>>> 
>>> getcallargs(f, 1, a=2)
Traceback (most recent call last):
    ...
TypeError: f() takes no arguments (2 given)

There is a comment in the patch about this case: the message given by Python is also incorrect, because it says that 2 positional arguments are given, but there was only 1 positional argument (the other was a keyword argument).  The patch currently handles this case by producing the same (incorrect) message as Python.