module__doc__: Document the Warning subclass heirarchy.

make_class(): Added a "goto finally" so that if populate_methods()
fails, the return status will be -1 (failure) instead of 0 (success).

fini_exceptions(): When decref'ing the static pointers to the
exception classes, clear out their dictionaries too.  This breaks a
cycle from class->dict->method->class and allows the classes with
unbound methods to be reclaimed.  This plugs a large memory leak in a
common Py_Initialize()/dosomething/Py_Finalize() loop.