Drop python 3.7 support & Remove tf-dataflow

[adason]

Peer Review Information

The package tornado is introduced as the dependencies, which seems to create circular dependencies when poetry tries to parse the dependency graph. This change seems to resolve the issue by dropping support for python 3.7.

Also, tf-dataflow is removed. This is not maintained and not in the critical path.

Pull Request Check List

• [ ] Added tests for changed code.
• [ ] Updated documentation for changed code.

[adason]

@Saurav-D What's the poetry version? Can you try poetry update in a virtual environment instead?