Migrate to pyproject.toml

[AlexHeide]

It looks like it's time to migrate from setup.py to pyproject.toml.

The conda-forge linter is requesting that we specify a pip backend, which we can't do for the legacy backend used for setup.py. This indicates that pip can encounter errors when looking for dependencies with the old setup.py setup_requires field.

[AlexHeide]

Along with this, we should add our pytest markers to pyproject.toml. They aren't currently visible without digging through the tests.