Use NEP 29 to determine supported Python and Numpy versions?

To determine Python and Numpy versions to be supported, how about following [NEP 29 — Recommend Python and Numpy version support as a community policy standard](https://numpy.org/neps/nep-0029-deprecation_policy.html)? It defines a drop schedule like this:

> ```
> On next release, drop support for Python 3.5 (initially released on Sep 13, 2015)
> On Jan 07, 2020 drop support for Numpy 1.14 (initially released on Jan 06, 2018)
> On Jun 23, 2020 drop support for Python 3.6 (initially released on Dec 23, 2016)
> On Jul 23, 2020 drop support for Numpy 1.15 (initially released on Jul 23, 2018)
> On Jan 13, 2021 drop support for Numpy 1.16 (initially released on Jan 13, 2019)
> On Jul 26, 2021 drop support for Numpy 1.17 (initially released on Jul 26, 2019)
> On Dec 22, 2021 drop support for Numpy 1.18 (initially released on Dec 22, 2019)
> On Dec 26, 2021 drop support for Python 3.7 (initially released on Jun 27, 2018)
> On Jun 21, 2022 drop support for Numpy 1.19 (initially released on Jun 20, 2020)
> On Apr 14, 2023 drop support for Python 3.8 (initially released on Oct 14, 2019)
> ```
>
> --- https://numpy.org/neps/nep-0029-deprecation_policy.html#drop-schedule

To implement this, I think we can:

* Mention in README that we use NEP 29.
* Print warning when using non-supported Python and Numpy versions.