Django 2.0a1 compat

[carltongibson]

Following from #5500, fixing failures against Django 2.0a1

[carltongibson]

@rpkilby There's a change in Django 2.0 which brings back issues around #4622. It's just a single failure now, but of course nothing about get_FOO_display in the release notes. Any thoughts?

[carltongibson]

@rpkilby They made get_FOO_display a functools.partial. Checking for this fixes the issue.

[rpkilby]

I'm trying to remember - is there a reason we don't just use a callable check there?

[rpkilby]

oh - right. Classes are also callables.

That said, I don't know if that really matters. Is there a reason to exclude classes here? Or other non-method/non-functions for that matter?

[carltongibson]

OK, this is green. Django2.0a1 is GO! 🎉

• I'll leave django20 as an allowed failure for now. We can tighten up nearer release.
• There's only a small number of failures on djangomaster — looks like the contrib.auth views are on the move.

This is where we wanted to be for today.

@rpkilby Thanks for the input!