add mypy workflow

.github/workflows/lint.yml

@@ -0,0 +1,26 @@
+name: Lint
+
+on:
+  push:
+    paths:
+      - '*.py'
+
+jobs:
+  mypy:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Setup Python
+        uses: actions/setup-python@v1
+        with:
+          python-version: 3.7.4
+          architecture: x64
+      - name: Checkout
+        uses: actions/checkout@v1
+      - name: Install mypy
+        run: pip install mypy
+      - name: Run mypy
+        uses: sasanquaneuf/mypy-github-action@releases/v1
+        with:
+          checkName: 'mypy'   # NOTE: this needs to be the same as the job name
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}