ci: don't forget to install mypy

.github/workflows/python-checks.yml

@@ -21,6 +21,7 @@ jobs:
       run: |
         python -m pip install --upgrade pip setuptools
         pip install -r requirements.txt -r dev-requirements.txt .
+        pip install mypy
     - name: Lint with flake8, mypy, etc.
       run: |
         flake8 olapy tests