Switch to PyMemcacheCache backend

The new library is already supported in django 3.2 and gives us better
support for IPv6 addresses and encrypted connections.

Also, the same library is being used by all other OpenStack projects.

Depends-on: https://review.opendev.org/c/openstack/devstack/+/898302
Closes-Bug: #2039225
Change-Id: I964ac4d0d62dff4c1f7c1f1373763fbf23024269

openstack_dashboard/local/local_settings.py.example

@@ -95,7 +95,7 @@ SECRET_KEY = secret_key.generate_or_read_from_file(
 # https://docs.djangoproject.com/en/1.11/topics/http/sessions/.
 #CACHES = {
 #    'default': {
-#        'BACKEND': 'django.core.cache.backends.memcached.MemcachedCache',
+#        'BACKEND': 'django.core.cache.backends.memcached.PyMemcacheCache',
 #        'LOCATION': '127.0.0.1:11211',
 #    },
 #}

openstack_dashboard/settings.py

@@ -172,7 +172,7 @@ MESSAGE_STORAGE = 'django.contrib.messages.storage.fallback.FallbackStorage'
 SESSION_ENGINE = 'django.contrib.sessions.backends.cache'
 CACHES = {
     'default': {
-        'BACKEND': 'django.core.cache.backends.memcached.MemcachedCache',
+        'BACKEND': 'django.core.cache.backends.memcached.PyMemcacheCache',
         'LOCATION': '127.0.0.1:11211',
     },
 }

releasenotes/notes/bug-2039225-39b7060a7e4bff07.yaml

@@ -0,0 +1,8 @@
+---
+upgrade:
+  - |
+    Default cache backend has been changed from MemcachedCache backend to
+    PyMemcacheCache backend. MemcachedCache backend was deprecated in django
+    3.2 and was later removed in django 4.1. Note that the LOCATION option
+    needs to be updated in case memcached servers use IPv6, so that inet6
+    prefix is not included.

test-requirements.txt

@@ -12,7 +12,7 @@ nodeenv>=0.9.4 # BSD
 pytest>=5.3.5 # MIT
 pytest-django>=3.8.0 # BSD (3 clause)
 pytest-html>=2.0.1 #MPL-2.0
-python-memcached>=1.59 # PSF
+pymemcache>=4.0.0  # Apache 2.0 License
 selenium>=2.50.1 # Apache-2.0
 testscenarios>=0.4 # Apache-2.0/BSD
 testtools>=2.2.0 # MIT