Use flake8 to find syntax errors and undefined names

lgtm.yml

@@ -35,3 +35,11 @@ extraction:
   python:
     python_setup:
       version: 3
+    after_prepare:
+      - python3 -m pip install --upgrade --user flake8
+    before_index:
+      - python3 -m flake8 --version  # flake8 3.6.0 on CPython 3.6.5 on Linux
+      # stop the build if there are Python syntax errors or undefined names
+      - python3 -m flake8 . --count --select=E901,E999,F821,F822,F823 --show-source --statistics
+      # exit-zero treats all errors as warnings.  The GitHub editor is 127 chars wide
+      - python3 -m flake8 . --count --exit-zero --max-complexity=10 --max-line-length=127 --statistics