fix: requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-TORNADO-5537286
- https://snyk.io/vuln/SNYK-PYTHON-TORNADO-5840803
- https://snyk.io/vuln/SNYK-PYTHON-TORNADO-6041512
- https://snyk.io/vuln/SNYK-PYTHON-TORNADO-7217828
- https://snyk.io/vuln/SNYK-PYTHON-TORNADO-7217829

requirements.txt

@@ -16,6 +16,6 @@ semver==2.10.1
 six==1.15.0 ; python_version >= '2.7' and python_version not in '3.0, 3.1, 3.2, 3.3'
 smmap==3.0.4 ; python_version >= '2.7' and python_version not in '3.0, 3.1, 3.2, 3.3'
 toml==0.10.1
-tornado==6.3.2 ; python_version >= '2.7' and python_version not in '3.0, 3.1, 3.2, 3.3'
+tornado==6.4.1 ; python_version >= '2.7' and python_version not in '3.0, 3.1, 3.2, 3.3'
 uritools==3.0.0 ; python_version ~= '3.5'
 wheel==0.34.2 ; python_version >= '2.7' and python_version not in '3.0, 3.1, 3.2, 3.3, 3.4'