Adopt NEP 29 for Python version support for wheels

I recommend adopting NEP 29 to define how and when to drop support for older minor releases of Python in the wheels.

In short it recommends supporting only those minor releases released within the 42 months prior to a release of this project. At time of writing that means Python 3.7 and above, but recommends dropping Python 3.7 at the end of the calendar year 2021.

This policy has already been adopted by Numpy itself, as well as Matplotlib, Astropy, and ligo.skymap, and conda-forge and the IGWN Conda Distribution.

/cc @leo-singer, @karl-wette