Fixed #25501 -- Made the file-based cache backend use the highest pickling protocol.

tests/cache/tests.py

@@ -65,6 +65,11 @@ class Unpickable(object):
         raise pickle.PickleError()
 
 
+class UnpicklableType(object):
+    # Unpicklable using the default pickling protocol on Python 2.
+    __slots__ = 'a',
+
+
 @override_settings(CACHES={
     'default': {
         'BACKEND': 'django.core.cache.backends.dummy.DummyCache',
@@ -1221,6 +1226,10 @@ class FileBasedCacheTests(BaseCacheTests, TestCase):
         cache.set('foo', 'bar')
         os.path.exists(self.dirname)
 
+    def test_cache_write_unpickable_type(self):
+        # This fails if not using the highest pickling protocol on Python 2.
+        cache.set('unpickable', UnpicklableType())
+
 
 @override_settings(CACHES={
     'default': {