fix: requirements.txt to reduce vulnerabilities (#25)

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-LXML-2940874

Author: snyk-bot

requirements.txt

@@ -25,7 +25,7 @@ idna-ssl==1.1.0
 isort==4.3.21
 kiwisolver==1.1.0
 lazy-object-proxy==1.4.3
-lxml==4.6.5
+lxml==4.9.1
 matplotlib==3.1.2
 mccabe==0.6.1
 mcstatus==2.3.0