Bump django from 2.2 to 2.2.28

Bumps [django](https://github.com/django/django) from 2.2 to 2.2.28.
- [Release notes](https://github.com/django/django/releases)
- [Commits](https://github.com/django/django/compare/2.2...2.2.28

)

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

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

requirements.txt

-Django>=1.10,<2.2; python_version >= '3.4'
+Django>=1.10,<2.3; python_version >= '3.4'
 Pillow>=2.9.0
 apscheduler>=3.3.0
 #django-constance[database]>=2.1