Support from __future__ import division
for classes such as Extension that support operator/ and/or operator/=
and change unit tests to use it
Removed commented-out lines in tests/function.py

Several obvious checks were commented out because they were applied to cloned functions,
which don't support the extra methods of function subclasses. An alternative would be to
cast the clone to the appropriate type, but the current tests suffice.
Merge branch 'tickets/DM-83'