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.

Start with the README and the NEP 29 drop schedule linked in the issue. Identify the project's existing Python and NumPy version handling, then determine the warning behavior and documentation needed; done means supported versions follow NEP 29 and unsupported versions are clearly reported.

Scritto dal modello di indicizzazione a partire dal testo della issue.