fix: python/docs/requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-SPHINX-570772

Author: snyk-bot

python/docs/requirements.txt

@@ -1,5 +1,5 @@
 googleapis-common-protos==1.56.1
 jinja2==3.0.0
-sphinx==2.3.1
+sphinx==3.0.4
 sphinx_rtd_theme==0.4.3
 sphinxcontrib-napoleon==0.7