remove reference to python 3.13 in setup.cfg

[slaupster]

Description

85cc763 changed the supported python range to exclude 3.13 but left this entry in classifiers after it had been previously added.

related:
a6317b3 343f4f2

I'm not familiar with the reasons for the back and forth on this but it confused me that its mentioned in setup.cfg but not actually pip installable in python 3.13.

Types of change

Should be a noop, just tidying.

tick off all the boxes. [] -> [x] -->

• I confirm that I have the right to submit this contribution under the project's MIT license.
• I ran the tests, and all new and existing tests passed.
• My changes don't require a change to the documentation, or if they do, I've added all required information.