Bump django from 2.2.8 to 2.2.28

Bumps [django](https://github.com/django/django) from 2.2.8 to 2.2.28.
- [Release notes](https://github.com/django/django/releases)
- [Commits](django/django@2.2.8...2.2.28)

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

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

requirements.txt

@@ -3,7 +3,7 @@ chardet==3.0.4
 coverage==4.5.3
 coveralls==1.7.0
 dj-database-url==0.5.0
-Django==2.2.8
+Django==2.2.28
 django-cors-headers==3.0.2
 django-coverage-plugin==1.6.0
 django-settings-context-processor==0.2