Bump django from 2.2.18 to 2.2.21

Bumps [django](https://github.com/django/django) from 2.2.18 to 2.2.21.
- [Release notes](https://github.com/django/django/releases)
- [Commits](django/django@2.2.18...2.2.21)

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

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

Author: dependabot[bot]

requirements.txt

@@ -3,7 +3,7 @@ certifi==2019.9.11
 chardet==3.0.4
 cycler==0.10.0
 dj-database-url==0.5.0
-Django==2.2.18
+Django==2.2.21
 django-heroku==0.3.1
 googletrans==2.4.0
 gunicorn==19.9.0