Update django requirement from ==2.2.* to ==3.0

Updates the requirements on [django](https://github.com/django/django) to permit the latest version.
- [Release notes](https://github.com/django/django/releases)
- [Commits](django/django@2.2a1...3.0)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

requirements/dev.txt

@@ -1,4 +1,4 @@
 -e .
 -r test.txt
 pypandoc==1.4
-django==2.2.*
+django==3.0