Add support for Python 3.13

[mcsken]

Goals

Note

Now possible since moving away from coveralls, to use codecov which doesn't require any project dependencies to work.

• Upgrade to python 3.13.

Estimate: 0.5h

[roskakori]

Closing because we are not going to use coveralls, it's just too much of a hassle.

[mcsken]

Re-opening since we replaced coveralls with codecov in #35, which doesn't require any dependencies.