fix: requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-FONTTOOLS-14151621
- https://snyk.io/vuln/SNYK-PYTHON-TORNADO-14400977
- https://snyk.io/vuln/SNYK-PYTHON-TORNADO-14400978
- https://snyk.io/vuln/SNYK-PYTHON-TORNADO-14400979

Author: snyk-bot

requirements.txt

@@ -3,3 +3,5 @@ scikit-image
 scipy
 autopep8
 bash_kernel
+fonttools>=4.61.0 # not directly required, pinned by Snyk to avoid a vulnerability
+tornado>=6.5.3 # not directly required, pinned by Snyk to avoid a vulnerability