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pip still relies on imp (due to cross-version compatibility requirements), so it may trigger a pending deprecation warning, and potentially return a non-zero return code:
http://buildbot.python.org/all/builders/AMD64%20Ubuntu%20LTS%203.x/builds/3142/steps/test/logs/stdio 

Reviewing the current buildbot failures, this is still the only one reporting the error:
http://buildbot.python.org/all/builders/AMD64%20Ubuntu%20LTS%203.x/builds/3165/steps/test/logs/stdio
I can reproduce the failure locally by running:
$ PYTHONWARNINGS=d ./python -m test test_venv
[1/1] test_venv
/home/ncoghlan/devel/py3k/Lib/imp.py:32: PendingDeprecationWarning: the imp module is deprecated in favour of importlib; see the module's documentation for alternative uses
 PendingDeprecationWarning)
/tmp/tmpwmsqtb9y/pip-1.5.rc1-py2.py3-none-any.whl/pip/wheel.py:127: DeprecationWarning: This method will be removed in future versions. Use 'parser.read_file()' instead.
test test_venv failed -- Traceback (most recent call last):
 File "/home/ncoghlan/devel/py3k/Lib/test/test_venv.py", line 289, in test_with_pip
 self.assertEqual(err, b"")
AssertionError: b"/home/ncoghlan/devel/py3k/Lib/imp.py:32:[160 chars]g)\n" != b''
1 test failed:
 test_venv
I'm updating the test to make sure that venv runs the subprocess in isolated mode.

New changeset 0ce8d68181a2 by Nick Coghlan in branch 'default':
Close #19694: venv now runs ensurepip in isolated mode
http://hg.python.org/cpython/rev/0ce8d68181a2 

Are you sure the problem is fixed? I still see the same error on the same buildbot.
http://buildbot.python.org/all/builders/AMD64%20Ubuntu%20LTS%203.x/builds/3168/steps/test/logs/stdio 

Using isolated mode addressed the warning noise, issue 19734 covers additional environment dependent problems in the test.