Add "Lint with flake8" step to detect Python syntax errors

These core flake8 tests can quickly find Python syntax errors and undefined names.

ci/python-package.yml

@@ -21,6 +21,13 @@ jobs:
       run: |
         python -m pip install --upgrade pip
         pip install -r requirements.txt
+    - name: Lint with flake8
+      run: |
+        pip install flake8
+        # 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: |
         pip install pytest