Add type annotations and mypy CI job

.github/workflows/python-package.yml

@@ -56,3 +56,13 @@ jobs:
       with:
         name: coverage-html
         path: htmlcov/*
+
+  static_type_check:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - uses: actions/setup-python@v2
+      with:
+        python-version: "3.12"
+    - run: python -m pip install mypy types-setuptools
+    - run: mypy