Fix broken graphviz import

Update introduction message in README.md and docs/index.rst

Upgrade dependencies and update runtime configs

Add support for another list representation, Node.equals and Node.clo…

…ne methods

Refactor code for readability and conciseness

Add type annotations

Add changes for version 6.0.0

Remove a setup.py classifier

Change Node.value to Node.val and minor refactors

Update README.rst and documentation