Bump django from 2.2.18 to 2.2.20 in /requirements (#3386)

Bumps [django](https://github.com/django/django) from 2.2.18 to 2.2.20.
- [Release notes](https://github.com/django/django/releases)
- [Commits](https://github.com/django/django/compare/2.2.18...2.2.20)

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

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

requirements/requirements.txt

@@ -1,4 +1,4 @@
-django==2.2.18
+django==2.2.20
 django-haystack==2.8.1
 django-filter==2.0.0
 djangorestframework==3.11.2