Add CI workflow

.github/workflows/ci.yml

@@ -0,0 +1,39 @@
+name: CI
+
+on:
+  pull_request:
+  push:
+    branches:
+      - main
+
+permissions:
+  contents: read
+
+concurrency:
+  group: ci-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  python:
+    name: Python checks
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Check out repository
+        uses: actions/checkout@v4
+
+      - name: Set up Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: "3.12"
+
+      - name: Check Python syntax
+        run: python -m compileall -q src
+
+      - name: Run unit tests if present
+        run: |
+          if [ -d tests ]; then
+            python -m unittest discover -s tests
+          else
+            echo "no tests directory yet"
+          fi