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/dynload_dl.c

@@ -12,11 +12,12 @@ extern char *Py_GetProgramName(void);
 const char *_PyImport_DynLoadFiletab[] = {".o", NULL};
 
 
-dl_funcptr _PyImport_GetDynLoadFunc(const char *shortname,
-                                    const char *pathname, FILE *fp)
+dl_funcptr _PyImport_FindSharedFuncptr(const char *prefix,
+                                       const char *shortname,
+                                       const char *pathname, FILE *fp)
 {
     char funcname[258];
 
-    PyOS_snprintf(funcname, sizeof(funcname), "PyInit_%.200s", shortname);
+    PyOS_snprintf(funcname, sizeof(funcname), "%20s_%.200s", prefix, shortname);
     return dl_loadmod(Py_GetProgramName(), pathname, funcname);
 }