Add more checks

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

.github/workflows/cargo_audit.yml

@@ -0,0 +1,24 @@
+name: Security audit
+on:
+  schedule:
+    - cron: '0 0 * * *'
+jobs:
+  audit:
+    runs-on: ubuntu-18.04
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions-rs/toolchain@v1
+        with:
+          toolchain: nightly
+          target: thumbv7em-none-eabi
+          override: true
+      - uses: actions/setup-python@v1
+        with:
+          python-version: 3.7
+      - name: Install Python dependencies
+        run: python -m pip install --upgrade pip setuptools wheel
+      - name: Set up OpenSK
+        run: ./setup.sh
+      - uses: actions-rs/audit-check@v1
+        with:
+          token: ${{ secrets.GITHUB_TOKEN }}