fix django-stubs version

.github/workflows/main.yml

@@ -99,7 +99,7 @@ jobs:
         run: |
           pip install -r requirements_test.txt
           pip install flake8 isort black mypy django-stubs dj_database_url types-six types-requests types-mock
-          pip install "django-stubs<1.13.0"  # Remove this line once https://github.com/typeddjango/django-stubs/issues/1227 is fixed
+          pip install "django-stubs<1.12.0"  # Remove this line once https://github.com/typeddjango/django-stubs/issues/1227 is fixed
           python setup.py develop
           pip install -e .
       - name: Running Flake8