Adopt NEP 29

[seanlaw]

NEP 29 proposes:

This NEP recommends that all projects across the Scientific Python ecosystem adopt a common “time window-based” policy for support of Python and NumPy versions. Standardizing a recommendation for project support of minimum Python and NumPy versions will improve downstream project planning.

https://numpy.org/neps/nep-0029-deprecation_policy.html

A detailed description is here:

https://numpy.org/neps/nep-0029-deprecation_policy.html#detailed-description

[seanlaw]

This will help ensure that our CI testing isn't forever growing as the Python version increases every 12-18 months.