[PR #7776/e9f7f7e2 backport][stable-6] CI: python-memcached 1.60 needs Python 3.6+ (#7777)

CI: python-memcached 1.60 needs Python 3.6+ (#7776)

python-memcached 1.60 needs Python 3.6+.

(cherry picked from commit e9f7f7e2de)

Co-authored-by: Felix Fontein <felix@fontein.de>

tests/unit/requirements.txt

@@ -6,7 +6,8 @@ unittest2 ; python_version < '2.7'
 importlib ; python_version < '2.7'
 
 # requirement for the memcached cache plugin
-python-memcached
+python-memcached < 1.60 ; python_version < '3.6'
+python-memcached ; python_version >= '3.6'
 
 # requirement for the redis cache plugin
 redis