[BUG] 0.4.6 breaks memcached integration

[jGleitz]

Upgrading from 0.4.5 to 0.4.6 breaks configurations that set up memcached for mailman-web like so:

CACHES = {
    'default': {
        'BACKEND': 'django.core.cache.backends.memcached.MemcachedCache',
        'LOCATION': os.environ.get('MEMCACHED_HOST', 'memcached') + ':11211',
    }
}

The reason seems to be that the python-memcached integration was dropped with django 3.2 (it’s documented for 3.1, but not for 3.2). django-mailman3 1.3.11 requires at least django 3.2.

To fix this, the image should install either pylibmc or pymemcache instead of python-memcached. Users can then switch to the corresponding django backend.

[jGleitz]

I took this opportunity to remove memcached caching from my setup entirely since it doesn’t really seem to be necessary since #463 was closed.

So personally, I would be fine with removing python-memcached from the container without replacement. I don’t know how many other users are relying on it, though.