Fixed #18770 -- memcached cache backend expects byte strings as keys

Thanks thecore for the report.

django/core/cache/backends/memcached.py

@@ -6,6 +6,7 @@ from threading import local
 from django.core.cache.backends.base import BaseCache, InvalidCacheBackendError
 
 from django.utils import six
+from django.utils.encoding import smart_str
 
 class BaseMemcachedCache(BaseCache):
     def __init__(self, server, params, library, value_not_found_exception):
@@ -50,6 +51,10 @@ class BaseMemcachedCache(BaseCache):
             timeout += int(time.time())
         return int(timeout)
 
+    def make_key(self, key, version=None):
+        # Python 2 memcache requires the key to be a byte string.
+        return smart_str(super(BaseMemcachedCache, self).make_key(key, version))
+
     def add(self, key, value, timeout=0, version=None):
         key = self.make_key(key, version=version)
         return self._cache.add(key, value, self._get_memcache_timeout(timeout))