Azrue pipelines: Use flake8 to find undefined names

azure-pipelines.yml

@@ -26,6 +26,11 @@ steps:
     pip install -r requirements.txt
   displayName: 'Install dependencies'
 
+- script: |
+    pip install flake8
+    flake8 . --count --exclude=./requests3/core/_http/packages/six.py --select=F821 --show-source --statistics
+  displayName: 'flake8 undefined names'
+
 - script: |
     pip install pytest pytest-azurepipelines
     pytest