Fixed a segfault on help(pya.Box)

Needed to refactor the class hierarchy of the Python classes.
Basically the module specific base class was removed as it does
not provide any benefit. The object layout of the PyObject
specialization was modified such that the payload is attached
to the end. This is compatible with the hidden extensions
which Python adds to normal objects.

testdata/python/basic.py

@@ -2718,6 +2718,7 @@ class BasicTest(unittest.TestCase):
     go = None
     self.assertEqual(pya.GObject.g_inst_count(), gc)
 
+
 # run unit tests
 if __name__ == '__main__':
   suite = unittest.TestSuite()