Following r26470, we update the version of setuptools used in Slicer, as a side effect you may get an error like the one reported below.
To address the problem
cd Slicer-Superbuild/python-setuptools git reset --hard HEAD
error: Your local changes to the following files would be overwritten by checkout: setup.cfg Please, commit your changes or stash them before you can switch branches. Aborting CMake Error at /home/jcfr/Projects/Slicer-2-Qt5-VTK8-build/python-setuptools-prefix/tmp/python-setuptools-gitupdate.cmake:147 (message): Failed to checkout tag: