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>

Author: dependabot[bot]

requirements.txt

@@ -1,3 +1,3 @@
-Django==2.2.20
+Django==2.2.21
 Pillow==8.1.1
 pytz==2020.1