Fixed #21351 -- Replaced memoize with Python's lru_cache.

Replaced the custom, untested memoize with a similar decorator from Python's
3.2 stdlib. Although some minor performance degradation (see ticket), it is
expected that in the long run lru_cache will outperform memoize once it is
implemented in C.

Thanks to EvilDMP for the report and Baptiste Mispelon for the idea of
replacing memoize with lru_cache.

tests/decorators/tests.py

@@ -1,5 +1,6 @@
 from functools import wraps
 from unittest import TestCase
+import warnings
 
 from django.contrib.admin.views.decorators import staff_member_required
 from django.contrib.auth.decorators import login_required, permission_required, user_passes_test
@@ -58,11 +59,14 @@ full_decorator = compose(
     staff_member_required,
 
     # django.utils.functional
-    lambda f: memoize(f, {}, 1),
     allow_lazy,
     lazy,
 )
 
+# suppress the deprecation warning of memoize
+with warnings.catch_warnings(record=True):
+    fully_decorated = memoize(fully_decorated, {}, 1)
+
 fully_decorated = full_decorator(fully_decorated)