Fix Python 3.4 cpp implementation

Fixes the ScalarMapContainer/MessageMapContainer implementations on
Python 3.4, by dynamically allocating their PyTypeObjects using
PyType_FromSpecWithBases, instead of statically allocating them. This is
necessary because Python 3.4+ disallows statically allocating a class
with a dynamically allocated parent.

Signed-off-by: Dan O'Reilly <oreilldf@gmail.com>