add mypy GH Action static type check configuration

Author: chrissimpkins

.github/workflows/py-typecheck.yml

@@ -0,0 +1,22 @@
+name: Python Type Checks
+
+on: [push]
+
+jobs:
+  build:
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        os: [ubuntu-latest]
+        python-version: [3.8]
+
+    steps:
+      - uses: actions/checkout@v2
+      - name: Set up Python ${{ matrix.python-version }}
+        uses: actions/setup-python@v2
+        with:
+          python-version: ${{ matrix.python-version }}
+      - name: Install Python testing dependencies
+        run: pip install --upgrade mypy
+      - name: Static type checks
+        run: mypy lib/fdiff