Planify Python version support.

[Carreau]

As discussed today this is an issue to discuss future support for multiple Python versions.

Currently Python 3.5 does not support many of the newer syntax feature and from the discussion there does not seem to be strong reasons to keep Python 3.5. Thus it seem like giving the Zarr community notice that we are planning to drop Python 3.5 support, give a few weeks to let users express concern, grievances, and if no objection, do one last release supporting 3.5.

In addition to this I suggest to adopt a schedule of dropping Python support ahead of time to give users time to planify. As we are just stopping support for 3.5 I suggest using NEP 29 but shifted by 12 month.

On next release, drop support for Python 3.5 (initially released on Sep 13, 2015), and support for Numpy 1.14 (initially released on Jan 06, 2018)
On Jun 23, 2021 drop support for Python 3.6 (initially released on Dec 23, 2016)
On Jul 23, 2021 drop support for Numpy 1.15 (initially released on Jul 23, 2018)
On Jan 13, 2022 drop support for Numpy 1.16 (initially released on Jan 13, 2019)
On Jul 26, 2022 drop support for Numpy 1.17 (initially released on Jul 26, 2019)
On Dec 26, 2022 drop support for Python 3.7 (initially released on Jun 27, 2018)
On Apr 14, 2024 drop support for Python 3.8 (initially released on Oct 14, 2019)

[jakirkham]

Would be good to get some thoughts from downstream users on this. CC'd a few below. Feel free to pull in others that may have thoughts here 🙂

cc @rabernat @jhamman @jni @sofroniewn

[jni]

For both napari and scikit-image we are following NEP-29 (plus a few months where convenient) so this would be completely fine by us.

[shoyer]

Xarray already requires Python 3.6, and we are considering requiring Python 3.7 soon.

[dstansby]

zarr is signed up to https://scientific-python.org/specs/spec-0000/, so I guess this could be closed?