Since inspect.isfunction(obj) is a precondition for calling

Since inspect.isfunction(obj) is a precondition for calling
inspect.getargspec(obj), test isfunction() directly in pydoc.py instead
of trying to indirectly deduce isfunction() in pydoc by virtue of
failing a combination of other tests.  This shouldn't have any visible
effect, except perhaps to squash a TypeError death if there was some path
thru this code that was inferring isfunction() by mistake.