Tornado-sqlalchemy

- Support SQLAlchemy 2.x (thanks [aemitos])

- Support for multiple databases (thanks [dingyaguang117])
- Drop `session_factory` and `make_session_factory` in favor of the new
`SQLAlchemy` instance
- Drop `declarative_base` in favor of the new `SQLAlchemy.Model` property
- Allow models to be associated to separate databases using `__bind_key__`

- [Please refer to [UPGRADING.md] for help with upgrading.]

- Pass `SessionMixin` arguments directly to `create_engine`
- Format code using `black`
- Add test cases for concurrent session usage

- Avoid initializing the pool executor at import time
- Drop support for Python 3.3 and 3.4
- Improve documentation and example code

- Update README

- Drop support for Python 2
- Extend `SessionMixin` to suport `self.session`
- Use `__call__` pattern on `declarative_base`
- Add more test cases
- Improve documentation and example code