fix: requirements-dev.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-TORNADO-5840803

Author: snyk-bot

requirements-dev.txt

@@ -23,3 +23,4 @@ selenium
 setuptools >= 65.5.1
 twine
 wheel >= 0.38.0
+tornado>=6.3.3 # not directly required, pinned by Snyk to avoid a vulnerability