Use App Engine-provided libraries where possible

Google now supports newer versions of both base Python (2.7.12 in the "classic" App Engine that I use) and several libraries. We should use supported built-in libraries where possible, for example for pytz (version 2016.4). We already use a built-in version of Django, but we could upgrade from 1.5 to a newer version such as 1.9. More information about using libraries in Google App Engine can be found here.