fix: requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-FLASKSECURITYTOO-6139926

requirements.txt

@@ -28,7 +28,7 @@ Flask-Login==0.5.0
 Flask-Mail==0.9.1
 Flask-Migrate==2.5.3
 Flask-Principal==0.4.0
-Flask-Security-Too==4.1.0
+Flask-Security-Too==5.3.3
 Flask-SocketIO==5.1.1
 Flask-SQLAlchemy==2.4.4
 Flask-WTF==0.14.3