Update gunicorn requirement from <20.0,>=19.9.0 to >=19.9.0,<21.0

Updates the requirements on [gunicorn](https://github.com/benoitc/gunicorn) to permit the latest version.
- [Release notes](https://github.com/benoitc/gunicorn/releases)
- [Commits](benoitc/gunicorn@19.9.0...20.1.0)

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

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

requirements.txt

@@ -8,7 +8,7 @@ django-cors-headers>=3.11.0,<4.0
 djangorestframework>=3.11.0,<4.0
 django-rest-knox>=4.1.0,<5.0
 django-filter>=2.2.0,<3.0
-gunicorn>=19.9.0,<20.0
+gunicorn>=19.9.0,<21.0
 importlib-metadata==4.13.0
 lxml>=4.6.3,<5.0
 markdown>=3.2.2,<4.0