[misc] Add flake8 linter to github actions

.github/workflows/backend.yml

@@ -20,6 +20,12 @@ jobs:
       run: |
         python -m pip install --upgrade pip
         pip install pytest flake8
+    - name: Lint with flake8
+      run: |
+        # stop the build if there are Python syntax errors or undefined names
+        flake8 . --count --select=E9,F63,F7,F82 --show-source --statistics
+        # exit-zero treats all errors as warnings. The GitHub editor is 127 chars wide
+        flake8 . --count --exit-zero --max-complexity=10 --max-line-length=127 --statistics
     - name: Test with pytest
       run: |
         pytest