Switched setup.py to setuptools.
Skipped inclusion of scripts (pip will take care of that).
Changed setup.py to use find_packages.
Merge pull request #2116 from apollo13/setuptools

 Switched setup.py to setuptools.
Fixed #18907 -- Correct docs regard population of model backrefs.

Thanks simonpercivall for the report and Aymeric for the patch.
Implement swappable model support for migrations
Fixed #21323 -- Improved readability of serialized Operation.