Closes SF patch 553277.  Per GvR, reverting to original patch -- the way to test if 'callable' has not been supplied is to test for None instead of False.  The previous correction to 'if callable()' was wrong because an unusable callback would be ignored rather than raising an exception.
Closes SF patch 553277.  Per GvR, reverting to original patch -- the way to
test if 'callable' has not been supplied is to test for None instead of
False.  The previous correction to 'if callable()' was wrong because an unusable
callback would be ignored rather than raising an exception.

sk-/python2.7-type-annotator,sk-/python2.7-type-annotator,sk-/python2.7-type-annotator