Bump django from 2.2.20 to 2.2.21

Bumps [django](https://github.com/django/django) from 2.2.20 to 2.2.21.
- [Release notes](https://github.com/django/django/releases)
- [Commits](django/django@2.2.20...2.2.21)

---
updated-dependencies:
- dependency-name: django
  dependency-type: direct:production
...

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

requirements.txt

@@ -1,5 +1,5 @@
 coverage==5.5
-Django==2.2.20
+Django==2.2.21
 django-debug-toolbar==3.2.1
 docutils==0.17.1
 freezegun==1.1.0