Support the latest sphinx and python versions

[choldgraf]

Sphinx is about to come out with a new release. We should make sure that our tests and supported versions in metadata support this release as well.

• add test cases for the latest python releases and drop testing for older versions (let's follow the scipy Supported version standards)
• do the same for sphinx
• update our metadata accordingly

[bsipocz]

For the sphinx incompatibility we have this issue here #681, which I hope gets resolved when the parser becomes sphinx9 compatible (there is a PR about it already).

[bsipocz]

for python 3.14, I already started #704 but didn't have time to dive down into the rabbit hole. IMO it's no acual incompatibilities but just cosmetics and we need to update the testing with newer mpl.