fix: requirements_dev.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-TORNADO-8400708

Author: snyk-bot

requirements_dev.txt

@@ -8,4 +8,4 @@ nbsphinx
 bump2version
 twine
 mock
-tornado>=6.3.3  # not directly required, pinned by Snyk to avoid a vulnerability
+tornado>=6.4.2  # not directly required, pinned by Snyk to avoid a vulnerability