bpo-30547: Fix multiple reference leaks (#1995)

Fix regressions introduced by:

- bpo-22257: commits 1abcf670 and 6b4be195Co-Authored-By: Victor Stinner <victor.stinner@gmail.com>
Co-Authored-By: Louie Lu <git@louie.lu>

Python/pylifecycle.c

@@ -302,9 +302,11 @@ initimport(PyInterpreterState *interp, PyObject *sysmod)
 
     /* Install importlib as the implementation of import */
     value = PyObject_CallMethod(importlib, "_install", "OO", sysmod, impmod);
-    if (value != NULL)
+    if (value != NULL) {
+        Py_DECREF(value);
         value = PyObject_CallMethod(importlib,
                                     "_install_external_importers", "");
+    }
     if (value == NULL) {
         PyErr_Print();
         Py_FatalError("Py_Initialize: importlib install failed");
@@ -325,6 +327,7 @@ initexternalimport(PyInterpreterState *interp)
         PyErr_Print();
         Py_FatalError("Py_EndInitialization: external importer setup failed");
     }
+    Py_DECREF(value);
 }