Compatibility backend for Python 3.7 (dev2)

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. Switch the builder from flit to setuptools. This change is intended to provide a second name of the compat backend package.
2. Split the version from __init__.py to version.py and add an extra package to allow setuptools parse the version without actually importing the module.
3. Make this compatible package only available for python=3.7.
4. Update the readme file to explain the purpose of this fork.

Full Changelog: v3.1.0.dev1-py37...v3.1.0.dev2-py37