Migrate from setup.py to pyproject.toml?

[weiji14]

Trying to do a bit of maintenance on this repo, and I'd like to reduce the number of configuration files in the top level folder if possible, by consolidating setup.py, setup.cfg, requirements.txt, requirements_test.txt -> pyproject.toml.

Notes:

• Since there was some resistance with switching away from versioneer in Add CI workflows #8, we'll also need to upgrade versioneer from 0.23 to 0.29 to support pyproject.toml. Edit: actually no, versioneer=0.23 works.
• The project currently uses flake8 which (infamously) doesn't support pyproject.toml. Might need to switch to ruff following Migrate to pyproject.toml, ruff, add build CI cf-xarray#400, or some alternative linter. Edit: See Migrate flake8, isort, black rules to ruff #49.

If I get some thumbs up 👍 on this, I'll open a PR to start the migration to pyproject.toml. Or if there are any objections, please raise them below.