Unpin Django in tests

It's not completely clear why this was pinned originally, though
at the time Jedi supported Python 2.7 as well as 3.5-3.8, so that
may have had something to do with it.

Removing this pin now seems to work in CI and unblocks some issues
we're seeing around Python 3.12 (specifically that Django<3.1
implicitly relies on distutils, which is no longer available by
default, and possibly other issues).

setup.py

@@ -41,7 +41,7 @@
               'docopt',
               # coloroma for colored debug output
               'colorama',
-              'Django<3.1',  # For now pin this.
+              'Django',
               'attrs',
           ],
           'qa': [