Fix Python3 binding's submodules

Previously the Python3 cv2 package ends up with no submodules (bgsegm, face,
etc) in it, which makes a lot of functionality unusable. By not writing over
our root reference we ensure the new submodules are added to the correct cv2
module.

modules/python/src2/cv2.cpp

@@ -1235,7 +1235,9 @@ static void init_submodule(PyObject * root, const char * name, PyMethodDef * met
         submod = PyImport_AddModule(full_name.c_str());
         PyDict_SetItemString(d, short_name.c_str(), submod);
     }
-    root = submod;
+
+    if (short_name != "")
+        root = submod;
   }
 
   // populate module's dict