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.

Python/importdl.h

@@ -8,8 +8,7 @@ extern "C" {
 
 extern const char *_PyImport_DynLoadFiletab[];
 
-extern PyObject *_PyImport_LoadDynamicModule(PyObject *name, PyObject *pathname,
-                                             FILE *);
+extern PyObject *_PyImport_LoadDynamicModuleWithSpec(PyObject *spec, FILE *);
 
 /* Max length of module suffix searched for -- accommodates "module.slb" */
 #define MAXSUFFIXSIZE 12