Drop upper bound for Django version

Since all Mailman Web really does is set up some urlpatterns and configuration, it will almost certainly work with any version of Django that Hyperkitty and Postorius work with. Despite this, it has its own version bound that hasn't been updated in step with Hyperkitty's and Postorius's. This means that users of Mailman Web are stuck on an obsolete version of Django for no reason.

We could just update Mailman Web's Django upper bound to match Postorius and Hyperkitty's, but given that it hasn't been kept up to date up til now, I think it's unlikely it will be kept up to date in the future. It's certainly a much higher risk than there being something in a new version of Django that breaks Mailman Web.