Add importlib.

Which is a python2.6-only requirement for kombu.  Since we're not yet
building the mirror on python2.6, we need to explicitly list it for
now.  Later we will add a mirror builder for each current OpenStack
testing platform.

Change-Id: I5254c5d7fdba8707a6f054ff613cea1619c7c0cc
Revert "Add importlib."

This reverts commit ec00b862b9963ed43242cf2765ccec2cb37f7872

This raises an exception if you try to install it in python2.7, which breaks the mirror script.

(And incidentally means that it must only be conditionally listed as a requirement for python2.6.)