python: fix a number of python initialization problems (bug #617587)

- pygtk.require("2.8") doesn't work - it's only after a major version,
  so we should pass in "2.0" instead
- init_pygobject() is deprecated, use pygobject_init (and pass in the
  version we require) instead
- init_pygtk() is a macro that returns from the current function on
  error, so we need to call it from a separate function for our error
  handling to work
- if some aspect of python initialization failed, we were still using
  the pygobject GIL macros, which were crashing