Testing and CI tidying including adding Python 3.12 job

[sadielbartholomew]

Minor admin tweaks to:

• add some missing newer Python versions into the CI test workflow, including support for the new latest major version 3.12, which was released about 3 weeks ago which I tested locally to be compatible dependency-wise with cf and that all tests pass as they should with it (:heavy_check_mark:);
• add some test method skips where SciPy is required under-the-hood but not directly in the test, which produces a somewhat confusing error message as an outcome of NameError: name 'convolve1d' is not defined. Did you mean: 'cf_convolve1d'? because it can't find the scipy convolve1d and suggests our own wrapper;
• make two small formatting tweaks so that test_style i.e. pycodestyle passes.

[davidhassell]

Thanks, Sadie - all these changes look very good to me. Please merge.