Bump django from 2.2.18 to 2.2.20

Bumps [django](https://github.com/django/django) from 2.2.18 to 2.2.20.
- [Release notes](https://github.com/django/django/releases)
- [Commits](django/django@2.2.18...2.2.20)

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

Author: dependabot[bot]

requirements.txt

@@ -1,3 +1,3 @@
-Django==2.2.18
+Django==2.2.20
 Pillow==8.1.1
 pytz==2020.1