Upgrade to Python 3.9

I also needed to patch my Jinja2 filters to not use a deprecated method
of declaring themselves, since the upgrade pulled in a newer minor
version of Jinja2.

The upgrade also tried to pull in Django 4, which many of the plugins
I'm using can't cope with yet, so it needed to be convinced not to do
that.

Pipfile

@@ -4,10 +4,10 @@ verify_ssl = true
 name = "pypi"
 
 [requires]
-python_version = '3.7'
+python_version = '3.9'
 
 [packages]
-django = "*"
+django = ">=3,<4"
 django-compressor = "*"
 gunicorn = {extras = ["gevent"]}
 "psycopg2-binary" = "*"
@@ -45,7 +45,7 @@ gevent = "*"
 django-extensions = "*"
 python-magic = "*"
 pyup-django = "*"
-"jinja2" = "*"
+jinja2 = "*"
 msgpack = "*"
 django-randomslugfield = "*"
 ago = "*"