This proposal and patch look good to me. The current behaviour definitely isn't desirable:

>>> import inspect
>>> inspect.Signature.from_function(1)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/ncoghlan/devel/py3k/Lib/inspect.py", line 1888, in from_function
    raise TypeError('{!r} is not a Python function'.format(func))
TypeError: 1 is not a Python function
>>> inspect.Signature.from_builtin(1)
>>> 

Signature.from_function() and Signature.from_builtin() should both also be documented, but we may want to wait for PEP 457 and #17373 in Python 3.5 before sorting all that out.