Upgrade/django 3.2

[VirginiaDooley]

This work upgrades dc_signup_form to Django 3.2 and addresses deprecation warnings.

[symroe]

This change means that it will only work with Django 3.2.0. We want a range here, starting at the lowest Django version we support (the first version to introduce re_path I expect?), and ending at <3.3 — if we use less than 3.3 then we get all minor version bumps after 3.2.0 and before 3.3.0.

You should also look at the CI matrix and add/remove the versions we want to support there.

[symroe]

Looks like Django 2.0 was the first to add re_path: https://docs.djangoproject.com/en/2.0/ref/urls/

[michaeljcollinsuk]

Think these two should be deleted now - as we won't be supporting below Django 2.0

[symroe]

I think we need at least one more line here. We have installs of Django 2.2 and (now) 3.2, so we should test them at least. It's also a good idea to test the next version of Django to see if one exists — that way we can catch errors in future versions early. So, we can have:

DJANGO_VERSION='>=2.2,<2.3' POSTGRES_VERSION=10
DJANGO_VERSION='>=3.2,<3.3' POSTGRES_VERSION=10
DJANGO_VERSION='>3.2' POSTGRES_VERSION=10

That gives us:

1. 2.2.X
2. 3.2.X
3. >3.2 (Django 4, but that's not out until December this year)