Integrate functionality from check-manifest
A common problem with Python packaging is creating and maintaining an accurate MANIFEST/MANIFEST.in file. It's easy to add a new submodule or piece of documentation, forget to include it in your MANIFEST, upload a release to PyPI, then have to issue a new point release to correct the problem.
check-manifest is a package that addresses this problem. If you run the command, it builds a source distribution and checks to see if there are files in the source directory that aren't in the package distribution; if there are missing files, it suggests MANIFEST.in rules that would correct the omission. It can also be used to generate an initial MANIFEST.in for a project.
In my opinion, check-manifest is sufficiently useful functionality that it warrants being part of setuptools itself.