improvement: deprecate requirements.txt and use Poetry for project dependency management.

`requirements.txt` is a loosely structured format that can be a bit of a pain to use. The project could use an improved dependency management scheme like `pyproject.toml` file offered by Poetry.
Other relevant discussion here:
https://discuss.python.org/t/examples-of-why-parsing-requirements-txt-files-to-feed-project-metadata-is-a-bad-idea/24828/2