"rety max must be less than or equal to 2", i got a little
aggressive.

django 2.2.0, django-guardian-2.2.0, new search labels, caching fixes

if the 'is_exposed' boolean is 'True', then the superevent is
public. changing the response so that it's 'is_public' would
require more than a two-line change.

Also, use Python 3.5 instead of Python 3.7 - I think we're going
to be using 3.5 for a bit since that's default for Debian 9.