Update gunicorn requirement from ~=19.7.1 to ~=20.0.4 (#220)

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@19.7.1...20.0.4)

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

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

Author: dependabot[bot]

requirements.txt

@@ -6,7 +6,7 @@ flask_sqlalchemy~=2.3.2
 psycopg2-binary~=2.8.6
 Flask-Migrate~=2.2.1
 pylint~=2.6.0
-gunicorn~=19.7.1
+gunicorn~=20.0.4
 csh_ldap~=2.1.0
 onesignal-sdk~=1.0.0
 pylint-quotes~=0.2.1