django 4 support: Add missing dependency for memcache

This adapts the memcached import the same way the swh.web module did.

Follow up 9210c012

Refs. swh/infra/sysadm-environment#5129

requirements-server.txt

@@ -3,3 +3,5 @@ djangorestframework
 psycopg2
 setuptools
 xmlschema
+pymemcache
+