CI: Setup github actions

This uses "setup.py test" to check if all boards are importable, until
a better solution is found or actual tests are added.

.github/workflows/main.yml

@@ -0,0 +1,13 @@
+on: [push, pull_request]
+name: CI
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check out source code
+        uses: actions/checkout@v2
+      - name: Set up Python
+        uses: actions/setup-python@v2
+      - name: Test
+        run: |
+          python setup.py test