…se Model classes, and other code modernization

- DB session binding based on SQLAlchemy 2, Base Model class declaration method change
- Reflected select, delete code based on SQLAlchemy 2
- Changed how to declare Model class based on SQLAlchemy 2
- Fixed issue with social connection termination
- Added pyproject.toml file after removing setup.py due to the introduction of PEP 517/518
- Fixed minimum installed version to Python 3.7

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