Remove _add_defaults_data_files override and add unittest #843
Reopening a pull request similar to PR #830. I've included unit test for that specific feature.
The only noticeable package that will fail with this patch is django_compressor because of dependencies on rcssmin and rjsmin. The main author has been contacted and is aware of the issue. It is best to organise a meaningful timeline with him to fix his issues before merging this PR.
@tweksteen: Would you consider augmenting this pull request to specifically detect the case where non-standard values are being passed for data_files, log an error, and omit the files? That way, for this rare case, setuptools will continue to work as it did before, but for everything else it will work fine. The warning message will also provide incentive to the offending packages to correct their behavior.
…n Python 2 where old style classes are used. Ref #843.