replace setup.py and setup.cfg with pyproject.toml

[raybellwaves]

I maintain the conda feedstock for this project and noticed some issues with the build.

https://dev.azure.com/conda-forge/feedstock-builds/_build/results?buildId=700914&view=logs&j=656edd35-690f-5c53-9ba3-09c10d0bea97&t=e5c8ab1d-8ff9-5cae-b332-e15ae582ed2d

There's a missing import I can probably fix but thought I would open an issue if there is interest in moving this project to use pyproject.toml

[mbarkhau]

I'm not completely opposed, but there is a build step performed by lib3to6 that may be an issue. So far, I have only successfully integrated it using setup.py.