add a GitHub actions workflow for python tox tests

this is adapted from
https://docs.github.com/en/actions/automating-builds-and-tests/building-and-testing-python#running-tests-with-tox

Author: Bomme

.github/workflows/main.yml

@@ -0,0 +1,23 @@
+name: Python Tests
+
+on: [push]
+
+jobs:
+  build:
+
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        python: ["3.6", "3.7", "3.8", "3.9"]
+
+    steps:
+      - uses: actions/checkout@v2
+      - name: Setup Python
+        uses: actions/setup-python@v2
+        with:
+          python-version: ${{ matrix.python }}
+      - name: Install tox and any other packages
+        run: pip install tox
+      - name: Run tox
+        # Run tox using the version of Python in `PATH`
+        run: tox -e py