setupext: Fix usage of ``reduce``.

``reduce`` was moved into ``functools`` in Python 3.  It has been
available there for some time (e.g., Python 2.6) so I switched to
importing it and falling back to the builtin if the import fails.
README: Add note about functools.reduce.
Merge pull request #4 from dave-shawley/setupext-py3-fix

Use functools.reduce in setupext