fix: requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-JINJA2-1012994

requirements.txt

@@ -27,7 +27,7 @@ idna==2.8                 # via requests
 imagesize==1.1.0          # via sphinx
 importlib-metadata==0.19  # via pluggy, pytest, tox
 itsdangerous==1.1.0       # via devpi-server
-jinja2==2.10.1            # via sphinx
+jinja2==2.11.3            # via sphinx
 markupsafe==1.1.1         # via jinja2
 mock==3.0.5
 more-itertools==5.0.0