fix: requirements.txt to reduce vulnerabilities

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

Author: snyk-bot

requirements.txt

@@ -4,3 +4,4 @@ scipy>=1.2.1
 matplotlib>=3.0.2
 xlrd>=1.2.0
 pillow>=10.3.0 # not directly required, pinned by Snyk to avoid a vulnerability
+fonttools>=4.61.0 # not directly required, pinned by Snyk to avoid a vulnerability