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

.circleci/config.yml

@@ -49,7 +49,7 @@ workflows:
   scheduled-workflow:
     triggers:
       - schedule:
-          cron: "@weekly"
+          cron: "0 0 * * *"
           filters:
             branches:
               only: master