Add MyPy action.

.github/workflows/mypy.yml

@@ -0,0 +1,19 @@
+name: Python type check
+on: [push, pull_request]
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v1
+      - name: Download submodules
+        run: git submodule update --init --recursive
+      - name: Set up Python
+        uses: actions/setup-python@v1
+        with:
+          python-version: 3.7
+      - name: Install dependencies
+        run: |
+          python -m pip install --upgrade pip
+          pip install mypy
+      - name: Lint with mypy
+        run: mypy -m lark || true