After a fresh install of rpm-ostree-toolbox, running "rpm-ostree-toolbox installer":

/usr/lib64/rpm-ostree-toolbox/py/rpmostreecompose/imagefactory.py:29: PyGIWarning: OSTree was imported without specifying a version first. Use gi.require_version('OSTree', '1.0') before import to ensure that the right version gets loaded.
  from gi.repository import Gio, OSTree, GLib  # pylint: disable=no-name-in-module
Traceback (most recent call last):
  File "/usr/lib64/rpm-ostree-toolbox/py/rpmostreecompose-main", line 22, in <module>
    from rpmostreecompose import imagefactory, installer, treecompose
  File "/usr/lib64/rpm-ostree-toolbox/py/rpmostreecompose/imagefactory.py", line 35, in <module>
    from imgfac.PersistentImageManager import PersistentImageManager
  File "/usr/lib/python2.7/site-packages/imgfac/PersistentImageManager.py", line 17, in <module>
    from ApplicationConfiguration import ApplicationConfiguration
  File "/usr/lib/python2.7/site-packages/imgfac/ApplicationConfiguration.py", line 26, in <module>
    from urlgrabber import urlopen
ImportError: No module named urlgrabber

After installing the python-urlgrabber package, this worked as expected.