Bump django from 2.2.16 to 2.2.28

Bumps [django](https://github.com/django/django) from 2.2.16 to 2.2.28.
- [Release notes](https://github.com/django/django/releases)
- [Commits](django/django@2.2.16...2.2.28)

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

Signed-off-by: dependabot[bot] <[email protected]>

requirements.txt

@@ -1,6 +1,6 @@
 # cryptography is required for heroku deployment
 coverage==3.6
-django==2.2.16
+django==2.2.28
 dj-database-url==0.2.1
 gunicorn==19.5.0
 psycopg2==2.8.6