Add the MakeHuman-head character head source (head=mh) with project eyes/brows/mouth and CC0 attribution

tools/tests/test_npc_parts.py

@@ -12,7 +12,7 @@ class CliTests(unittest.TestCase):
         self.assertEqual(
             set(npc_parts.PART_NAMES),
             {"head", "face", "hand_l", "hand_r", "head_proto",
-             "hand_l_proto", "hand_r_proto"},
+             "hand_l_proto", "hand_r_proto", "head_mh"},
         )
 
     def test_blender_commands_fail_cleanly_without_bpy(self):