Issue #24268: Address some PEP 489 refleaks

- missing DECREF in PyModule_FromDefAndSpec2
- missing DECREF in PyType_FromSpecAndBases2
- missing DECREF in _testmultiphase module

Patch by Petr Viktorin

Modules/_testmultiphase.c

@@ -262,6 +262,7 @@ createfunc_nonmodule(PyObject *spec, PyModuleDef *def)
         return NULL;
     }
     PyDict_SetItemString(dct, "three", three);
+    Py_DECREF(three);
 
     ns = _PyNamespace_New(dct);
     Py_DECREF(dct);