Add more checks

- python code formatting is tested with yapf
- cron workflow to run cargo audit
- Don't run on latest

.github/workflows/mdlint.yml

@@ -10,11 +10,12 @@ on:
 
 jobs:
   mdlint:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
     steps:
       - uses: actions/checkout@v2
       - name: markdownlint-cli
         uses: nosborn/github-action-markdown-cli@v1.1.1
         with:
           files: '**/*.md'
+          ignore_files: "third_party/*"
           config_file: ".markdownlint.json"