github: Use a github action for testing

Introduce a github action that will run tox on a number of python
versions and host OSes for testing.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>

.github/workflows/test.yml

@@ -0,0 +1,23 @@
+name: Python Test
+
+on: [push,pull_request]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        os: [ubuntu-latest, macos-latest, windows-latest]
+        python-version: [3.6, 3.7, 3.8]
+    steps:
+      - uses: actions/checkout@v1
+      - name: Set up Python
+        uses: actions/setup-python@v1
+        with:
+          python-version: ${{ matrix.python-version }}
+      - name: Display Python version
+        run: python -c "import sys; print(sys.version)"
+      - name: install tox
+        run: pip3 install tox
+      - name: tox
+        run: tox