Update mailman-web to use Python 3 (For rolling releases) (#217)

* Update mailman-web to use Python 3

* Update configuration

* Python 3.6 image.

* Use alpine 3.7

* Use new style middleware

* Use alpine 3.6

* In Python 3, urlparse was renamed to urllib.parse

web/docker-entrypoint.sh

@@ -16,7 +16,7 @@ function wait_for_postgres () {
 
 function wait_for_mysql () {
 	# Check if MySQL is up and accepting connections.
-	HOSTNAME=$(python -c "from urlparse import urlparse; o = urlparse('$DATABASE_URL'); print(o.hostname);")
+	HOSTNAME=$(python -c "from urllib.parse import urlparse; o = urlparse('$DATABASE_URL'); print(o.hostname);")
 	until mysqladmin ping --host "$HOSTNAME" --silent; do
 		>&2 echo "MySQL is unavailable - sleeping"
 		sleep 1