I think 

PyObject *PyInit_myextention(void)

should be

PyMODINIT_FUNC PyInit_myextention(void)

on https://docs.python.org/3/howto/cporting.html#module-initialization-and-state

It didn't work for me until I replaced this with a message in the about 'undefined PyInit_myextention'. However, when I used nm to inspect the .so object file, I fond the PyInit_myextention (but probably with the wrong return type). Moreover, whenever I would remove the same .so importing resulted in a different error complaining that the module does not exist (strongly suggesting that I did not mix up .so files).

Good luck!