fix: requirements/base.txt to reduce vulnerabilities (#308)

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-MARKDOWN2-1063233
- https://snyk.io/vuln/SNYK-PYTHON-MARKDOWN2-1321158
- https://snyk.io/vuln/SNYK-PYTHON-MARKDOWN2-1759306
- https://snyk.io/vuln/SNYK-PYTHON-MARKDOWN2-2606975
- https://snyk.io/vuln/SNYK-PYTHON-MARKDOWN2-567097
- https://snyk.io/vuln/SNYK-PYTHON-MARKDOWN2-568905

Co-authored-by: snyk-bot <snyk-bot@snyk.io>

Author: chenrui333

requirements/base.txt

@@ -4,4 +4,4 @@ jinja2==2.10.3
 click==7.0
 tqdm==4.15.0
 requests==2.22.0
-markdown2==2.3.8
+markdown2==2.4.3