Fix infinite recursion

[markusschmaus]

Code review checklist

• Pull request represents a single change (i.e. not fixing disparate/unrelated things in a single PR)
• Title summarizes what is changing
• Commit messages are meaningful (see this for details)
• Tests have been included and/or updated
• Docstrings have been included and/or updated, as appropriate
• Standalone docs have been updated accordingly
• Changelog(s) has/have been updated, as needed (see CHANGELOG.md, no need
  to update for typo fixes and such).

(Inception PR)

This fixes the infinite recursion we are getting when trying to call super suing type(obj). By defining the replacement methods inside a method each replacement is bound to the original class without needing to look it up.

[goto40]

Awesome! Can you add a small unittest?

[markusschmaus]

Most of this PR addresses a problem with python2 which led all tests on my end fail. I added a small test which tests the correct behavior in the case of attrs classes which inherit from each other, which is also fixed in this PR.

[markusschmaus]

I fixed this error:

E               ValueError: 

Non keyword-only attributes are not allowed after a keyword-only attribute

Newer versions of attrs (including the one I use) no longer have this restriction. python-attrs/attrs#559

[igordejanovic]

Thanks @markusschmaus, great work! This looks good to me.

With this change we can publish this in the next minor release and drop Python 2 in the next major release.

I noticed a leftover docstring so you can remove it. BTW, do you want to clean this branch yourself or should I squash merge?

[markusschmaus]

Squash merge sounds great to me