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.

Beginne mit der README und dem im Issue verlinkten NEP-29-Drop-Zeitplan. Ermittle die vorhandene Handhabung der Python- und NumPy-Versionen im Projekt und bestimme anschließend das Warnverhalten sowie die erforderliche Dokumentation. Die Aufgabe ist abgeschlossen, wenn die unterstützten Versionen dem NEP 29 folgen und nicht unterstützte Versionen eindeutig gemeldet werden.

Vom Indexierungsmodell aus dem Issue-Text verfasst.