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/staticfiles_tests/tests.py

@@ -55,7 +55,7 @@ class BaseStaticFilesTestCase(object):
         storage.staticfiles_storage._wrapped = empty
         # Clear the cached staticfile finders, so they are reinitialized every
         # run and pick up changes in settings.STATICFILES_DIRS.
-        finders._finders.clear()
+        finders.get_finder.cache_clear()
 
         testfiles_path = os.path.join(TEST_ROOT, 'apps', 'test', 'static', 'test')
         # To make sure SVN doesn't hangs itself with the non-ASCII characters
@@ -561,7 +561,7 @@ class TestCollectionCachedStorage(BaseCollectionTestCase,
         Test that post_processing indicates the origin of the error when it
         fails. Regression test for #18986.
         """
-        finders._finders.clear()
+        finders.get_finder.cache_clear()
         err = six.StringIO()
         with self.assertRaises(Exception):
             call_command('collectstatic', interactive=False, verbosity=0, stderr=err)
@@ -756,6 +756,15 @@ class TestMiscFinder(TestCase):
         self.assertRaises(ImproperlyConfigured,
             finders.get_finder, 'foo.bar.FooBarFinder')
 
+    def test_cache(self):
+        finders.get_finder.cache_clear()
+        for n in range(10):
+            finders.get_finder(
+                'django.contrib.staticfiles.finders.FileSystemFinder')
+        cache_info = finders.get_finder.cache_info()
+        self.assertEqual(cache_info.hits, 9)
+        self.assertEqual(cache_info.currsize, 1)
+
     @override_settings(STATICFILES_DIRS='a string')
     def test_non_tuple_raises_exception(self):
         """