Add mypy step in GitHub Actions

Author: RadoRado

.github/workflows/django.yml

@@ -24,6 +24,8 @@ jobs:
       run: |
         python -m pip install --upgrade pip
         pip install -r requirements/local.txt
+    - name: Type check
+      run: mypy styleguide_example/
     - name: Run migrations
       run: python manage.py migrate
     - name: Run tests