Temporarily switch PyPy jobs back to precisepy3k

The previous change to switch to py3k-precise nodepool nodes
overlooked that the old static precisepy3k slaves had on them cruft
Python 2.7 installs of setuptools which worked with pypy. Switch
back to them for now while we work on solving that issue.

Change-Id: I6899035d0c422f622699f6729f573d16e4a9bc91