github: add mypy workflow

This will run mypy only on changed files

.github/workflows/mypy.yml

@@ -0,0 +1,23 @@
+name: "mypy check"
+on: [pull_request]
+
+jobs:
+
+  static-type-check:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - uses: actions/setup-python@v3
+      with:
+        python-version: '3.x'
+    - run: pip install mypy
+    - name: Get Python changed files
+      id: changed-py-files
+      uses: tj-actions/changed-files@v23
+      with:
+        files: |
+          *.py
+          **/*.py
+    - name: Run if any of the listed files above is changed
+      if: steps.changed-py-files.outputs.any_changed == 'true'
+      run: mypy ${{ steps.changed-py-files.outputs.all_changed_files }}