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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.