Switch from coveralls to codecov

[liamhuber]

I will look at @jan-janssen's switch on the individual repos to see about making the same switch here in the centralized location. The driving force for this is coveralls not playing nicely with python 3.13 pyiron/pyiron_snippets#36.

This is not super high-priority though, so I don't expect to get here for at least a fortnight.