Run PyPy jobs on py3k-precise single-use slaves

The issues which caused us to undo this originally seemed to be
related to setuptools 2.1, and can no longer be reproduced with
setuptools 2.2, so this seems to be safe now... manually tested PyPy
unit test jobs for numerous projects on a held py3k-precise node
with no problem.

* .../jenkins_job_builder/config/python-jobs.yaml: Switch the node
label for gate-{name}-pypy jobs from the old static slaves to the
new single-use slaves managed by nodepool.

* .../zuul/layout.yaml: Stop setting reusable_node for gate-.*-pypy
jobs now that they use single-use nodes.

Change-Id: I393d27f7c928454bfb7c165ff26f192183ef6a23
Merge "Run PyPy jobs on py3k-precise single-use slaves"