Py3 final migration

[tBuLi]

There were a lot of fossils in the code from the time we still had to support both py27 and py3x. Those days are long gone, so it's time for a major clean-up. The key points of this PR are:

• Keyword-only arguments can just be used in the way Python intended. There is no longer a need for the keywordonly decorator.
• Remove all explicit sys.version_info >= (3, 0) checks, since the answer is always True.
• Use the built-in partial and delete our own implementation which mimicked the py3 partial function in py27.

Up for debate:

• Python has a built-in cached_property since python 3.8. However, if I swap our implementation for this, pickling breaks in a way that is not that is not obvious to fix. So not sure yet if this is worth the effort. The problem is that now we simply check if the class dict contains entries starting with _cached because those are, well, cached. But this no longer works with the built-in, so we would have to get smart, which is difficult.

[tBuLi]

Yeah I agree, we can leave cached_property as an issue.

Weirdly enough I am unable to reproduce the failing test for test_global_opt.py::TestGlobalOptGaussian::test_chained_min locally, even though I recreated the same environment on my own mac. Weirdly enough however, the error is very consistent in all macos tests in our build matrix it seems. Not sure what to do from here.

I agree on thinning out the build matrix btw, it seems a bit excessive.