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.

setup.py

@@ -620,6 +620,8 @@ class PyBuildExt(build_ext):
         exts.append( Extension('_testbuffer', ['_testbuffer.c']) )
         # Test loading multiple modules from one compiled file (http://bugs.python.org/issue16421)
         exts.append( Extension('_testimportmultiple', ['_testimportmultiple.c']) )
+        # Test multi-phase extension module init (PEP 489)
+        exts.append( Extension('_testmultiphase', ['_testmultiphase.c']) )
         # profiler (_lsprof is for cProfile.py)
         exts.append( Extension('_lsprof', ['_lsprof.c', 'rotatingtree.c']) )
         # static Unicode character database