CLN: Clean uses of super, part II
- [x] xref #25725 & #26177

Continuation from #26177. In addition to the uses of ``super`` dealt with in #26177, there were some more advanced uses of ``super`` in the code base, e.g. in classmethods and using ``super`` to jump to a specific point in the MRO-chain.

This PR specifically deals with:
* ``super(..., self)`` in pyx-files
* ``super(..., cls)`` in the whole code base
* Changes some specific uses of ``super``, se comments .
Hello @topper-123! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:





















There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2019-04-28 07:40:11 UTC