Change the Fini function to only remove otherwise unreferenced strings

from the interned table.  There are references in hard-to-find static
variables all over the interpreter, and it's not worth trying to get
rid of all those; but "uninterning" isn't fair either and may cause
subtle failures later -- so we have to keep them in the interned
table.

Also get rid of no-longer-needed insert of None in interned dict.

Objects/stringobject.c

@@ -1043,8 +1043,6 @@ PyString_InternInPlace(p)
 		interned = PyDict_New();
 		if (interned == NULL)
 			return;
-		/* Force slow lookups: */
-		PyDict_SetItem(interned, Py_None, Py_None);
 	}
 	if ((t = PyDict_GetItem(interned, (PyObject *)s)) != NULL) {
 		Py_INCREF(t);
@@ -1078,10 +1076,6 @@ void
 PyString_Fini()
 {
 	int i;
-#ifdef INTERN_STRINGS
-	Py_XDECREF(interned);
-	interned = NULL;
-#endif
 	for (i = 0; i < UCHAR_MAX + 1; i++) {
 		Py_XDECREF(characters[i]);
 		characters[i] = NULL;
@@ -1090,4 +1084,20 @@ PyString_Fini()
 	Py_XDECREF(nullstring);
 	nullstring = NULL;
 #endif
+#ifdef INTERN_STRINGS
+	if (interned) {
+		int pos, changed;
+		PyObject *key, *value;
+		do {
+			changed = 0;
+			pos = 0;
+			while (PyDict_Next(interned, &pos, &key, &value)) {
+				if (key->ob_refcnt == 2 && key == value) {
+					PyDict_DelItem(interned, key);
+					changed = 1;
+				}
+			}
+		} while (changed);
+	}
+#endif
 }