Compatibility backend for Python 3.7 (dev1)

This release pushes a backend package to

https://pypi.org/manage/project/flask-sqlalchemy-compat-backend-py37

for supporting the compatibility when using Python 3.7 and Flask-SQLAlchemy-compat 🔗.

This update

1. Make Flask-SQLalchemy as the main dependency to prevent reinstallation caused by other packages.
2. Update the version to refresh the PyPI package.

Full Changelog: v3.0.5-py37...v3.1.0.dev1-py37