Fix building Pate with Python 3.

Python's C API changed a lot between version 2 and 3, and in particular
Python 3.2 removed a lot of the old CObject API. At the moment
simultaneous compatibility is achieved by a series of #if blocks,
possibly some refactoring is in order.

Pate builds with both Python 2 and Python 3 now.

BUG: 310220
REVIEW: 107362