Consider dropping support for unsupported Pythons?

[movermeyer]

Things are getting harder to support both modern versions of Python alongside old ones.
This issue will be a place where I can document the relevant information, and any decisions going forward.

---

• Python 2 is no longer available in the Ubuntu Docker image
• pip and tox removed support for Python < 3.8

So as a result I don't update our benchmarking numbers for Python < 3.8 anymore.

• cimg/python doesn't support Python 3.4 (but we have a workaround)
• Python 3.12 dropped setuptools
• setuptools itself dropped test runner features

It might be time to give in and drop support for older versions.
Going forward, we might decide that we'll only support to versions of Python that are still supported by upstream.

Benefits

• We could eliminate some code by dropping support for Python < 3.9
• We'd be able to run our benchmarking for all supported versions
• We'd be able to move to a more modern Python packaging system (Test and build using PEP 517-compliant methods #147)

Detriments

• Some users wouldn't be able to use the latest versions of ciso8601:

Last month's (2024-10) download stats:

category	percent	downloads
3.14	0.00%	13
3.13	0.06%	2,708
3.12	3.69%	168,627
3.11	20.38%	930,445
3.10	40.81%	1,862,952
3.9	10.09%	460,599
3.8	8.93%	407,673
3.7	1.57%	71,752
3.6	4.24%	193,677
3.5	0.00%	9
3.4	0.00%	2
2.7	0.14%	6,582
null	10.07%	459,887
Total		4,564,926

So at least 14.88% of downloads are from unsupported versions (though 8.93% is from the very recently unsupported 3.8 so we might see that drop quickly in the coming months).

This would of course be a breaking change, so we'd have a major version bump for it.