GitHub Actions CI: find Python undefined names

Use [GitHub Actions CI](https://github.com/features/actions) to find undefined names in Python code like the ones that were fixed in dependabot#1219

Based on https://github.com/actions/starter-workflows/actions/new

.github/workflows/pythonpackage.yml

@@ -0,0 +1,26 @@
+name: Python package
+
+on: [pull_request, push]
+
+jobs:
+  build:
+
+    runs-on: ubuntu-latest
+    strategy:
+      max-parallel: 1
+      matrix:
+        python-version: [3.7]
+
+    steps:
+    - uses: actions/checkout@v1
+    - name: Set up Python ${{ matrix.python-version }}
+      uses: actions/setup-python@v1
+      with:
+        python-version: ${{ matrix.python-version }}
+    - 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