[INFRA] Switch over to pyproject.toml

[ericmjl]

Brief Description

pyproject.toml has become the de facto standard for declaring package metadata. I would like to propose that we move from our setup.py + requirements.txt pattern to one that involves just pyproject.toml.

To do this, we would probably have to do the following:

• Port everything from .requirements/ into pyproject.toml, and then delete the directory,
• Port everything from setup.py into pyproject.toml as well, and then delete the file.