Changes proposed in PEP3121 and PEP384 have now been applied to the xxsubtype module!

It looks like your patch will result in reference leaks if type creation fails.  I think you should add Py_DECREF(m) before error returns.

The PEP 3121 part is fixed by:

commit d5cacbb1d9c3edc02bf0ba01702e7c06da5bc318
Author: Nick Coghlan <ncoghlan@gmail.com>
Date:   Sat May 23 22:24:10 2015 +1000

    PEP 489: Multi-phase extension module initialization
    
    Known limitations of the current implementation:
    
    - documentation changes are incomplete
    - there's a reference leak I haven't tracked down yet
    
    The leak is most visible by running:
    
      ./python -m test -R3:3 test_importlib
    
    However, you can also see it by running:
    
      ./python -X showrefcount
    
    Importing the array or _testmultiphase modules, and
    then deleting them from both sys.modules and the local
    namespace shows significant increases in the total
    number of active references each cycle. By contrast,
    with _testcapi (which continues to use single-phase
    initialisation) the global refcounts stabilise after
    a couple of cycles.

See bpo-41111 "[C API] Convert a few stdlib extensions to the limited C API (PEP 384)" for the PEP 384 part.