[photonlibpy] add mypy to ci (#1570)

Co-authored-by: James Ward <james@thedropbears.org.au>

.github/workflows/python.yml

@@ -37,7 +37,7 @@ jobs:
       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip
-          pip install setuptools wheel pytest
+          pip install setuptools wheel pytest mypy
 
       - name: Build wheel
         working-directory: ./photon-lib/py
@@ -50,6 +50,13 @@ jobs:
             pip install --no-cache-dir dist/*.whl
             pytest
 
+      - name: Run mypy type checking
+        uses: liskin/gh-problem-matcher-wrap@v3
+        with:
+          linters: mypy
+          run: |
+            mypy --show-column-numbers --config-file photon-lib/py/pyproject.toml photon-lib
+
 
       - name: Upload artifacts
         uses: actions/upload-artifact@master