Add support for Python 3.2:

- Move ``_pickle.c`` -> ``_pickle_33.c``.

- Clone Python 3.2.3's ``_pickle.c`` -> ``_pickle_32.c`` and apply the
  same patch.

- Choose between them at build time based on ``sys.version_info``.

- Disable some tests of 3.3-only features.
Merge trunk.