As reported by @mdehoon:

I noticed that this release candidate will always skip the Mac OS X backend,
as the .check() method of BackendMacOSX in setupext.py returns None. Adding
return "darwin"
or some other string solves the issue.
Also, if I am not mistaken, the files under
lib/matplotlib/backends/Matplotlib.nib (as returned by the
get_package_data() method of BackendMacOSX in setupext.py) are needed
by the cocoaagg backend, not by the MacOSX backend.