fix: requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-LXML-16119103

Author: snyk-bot

requirements.txt

@@ -11,7 +11,7 @@ sphinxcontrib-websupport==1.2.7
 pip~=26.0
 build==0.10.0
 wheel~=0.46.2
-lxml==5.3.1
+lxml==6.1.0
 utils~=1.0.2
 keyring==23.13.1
 docutils==0.21.2