nixpkgs: z3 is now MIT licensed.

It's also been moved to GitHub, meaning we can avoid some of the
hackiness in the original expression. This updates the Git revision, but
only so that it contains the proper license (it's otherwise equivalent
to Z3 v4.3.2)

Also, make sure the python API .py files exist besides the .pyc files.

Signed-off-by: Austin Seipp <aseipp@pobox.com>