Install mypy in workflow action

.github/workflows/test_coverage.yml

@@ -20,6 +20,7 @@ jobs:
         python -m pip install --upgrade pip
         pip install flake8
         pip install coverage
+        pip install mypy
         if [ -f requirements.txt ]; then pip install -r requirements.txt; fi
     - name: Lint with flake8
       run: |