Fix 500 error when adding age ranges

See merge request !159

Django 2.2 and Python 3 upgrade

See merge request !157

Not really sure what has changed here, maybe exception chaining in
Python 3? I think we're only interested in the line number being reported
to the user.

This means they can be loaded in tests without loading a bunch of other stuff
that will cause things to break in tests.

response.content now bytes not str
terms are now single quoted. TODO: Is this OK?