Bump django from 2.2.6 to 2.2.20

Bumps [django](https://github.com/django/django) from 2.2.6 to 2.2.20.
- [Release notes](https://github.com/django/django/releases)
- [Commits](django/django@2.2.6...2.2.20)

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