fix: requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-SETUPTOOLS-3113904

Author: snyk-bot

requirements.txt

@@ -17,4 +17,5 @@ pyupgrade
 requests
 Sphinx
 sphinx_rtd_theme
-sphinx_mdinclude
+sphinx_mdinclude
+setuptools>=65.5.1 # not directly required, pinned by Snyk to avoid a vulnerability