Add GitHub Actions workflow for CI

.github/workflows/build.yml

@@ -0,0 +1,17 @@
+name: build
+on: [pull_request]
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v1
+    - name: Set up Python 3.7
+      uses: actions/setup-python@v1
+      with:
+        python-version: 3.7
+    - name: Install dependencies
+      run: pip install -r requirements.txt
+    - name: Run syntax check
+      run: python check.py
+    - name: Build site
+      run: python build.py