test.test_venv.EnsurePipTest.test_with_pip() fails with --without-threads, because some dependencies of PIP (cachecontrol, lockfile) unconditionally import threading module.

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FAIL: test_with_pip (test.test_venv.EnsurePipTest)
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Traceback (most recent call last):
  File "/tmp/cpython/Lib/test/test_venv.py", line 356, in test_with_pip
    with_pip=True)
subprocess.CalledProcessError: Command '['/tmp/tmpgt_wdmiq/bin/python', '-Im', 'ensurepip', '--upgrade', '--default-pip']' returned non-zero exit status 1

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/tmp/cpython/Lib/test/test_venv.py", line 362, in test_with_pip
    self.fail(msg.format(exc, details))
AssertionError: Command '['/tmp/tmpgt_wdmiq/bin/python', '-Im', 'ensurepip', '--upgrade', '--default-pip']' returned non-zero exit status 1

**Subprocess Output**
Traceback (most recent call last):
  File "/tmp/cpython/Lib/runpy.py", line 170, in _run_module_as_main
    "__main__", mod_spec)
  File "/tmp/cpython/Lib/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/tmp/cpython/Lib/ensurepip/__main__.py", line 4, in <module>
    ensurepip._main()
  File "/tmp/cpython/Lib/ensurepip/__init__.py", line 209, in _main
    default_pip=args.default_pip,
  File "/tmp/cpython/Lib/ensurepip/__init__.py", line 116, in bootstrap
    _run_pip(args + [p[0] for p in _PROJECTS], additional_paths)
  File "/tmp/cpython/Lib/ensurepip/__init__.py", line 40, in _run_pip
    import pip
  File "/tmp/tmp1bl0s_61/pip-7.1.2-py2.py3-none-any.whl/pip/__init__.py", line 15, in <module>
  File "/tmp/tmp1bl0s_61/pip-7.1.2-py2.py3-none-any.whl/pip/vcs/mercurial.py", line 10, in <module>
  File "/tmp/tmp1bl0s_61/pip-7.1.2-py2.py3-none-any.whl/pip/download.py", line 44, in <module>
  File "/tmp/tmp1bl0s_61/pip-7.1.2-py2.py3-none-any.whl/pip/_vendor/cachecontrol/__init__.py", line 9, in <module>
  File "/tmp/tmp1bl0s_61/pip-7.1.2-py2.py3-none-any.whl/pip/_vendor/cachecontrol/wrapper.py", line 1, in <module>
  File "/tmp/tmp1bl0s_61/pip-7.1.2-py2.py3-none-any.whl/pip/_vendor/cachecontrol/adapter.py", line 5, in <module>
  File "/tmp/tmp1bl0s_61/pip-7.1.2-py2.py3-none-any.whl/pip/_vendor/cachecontrol/controller.py", line 11, in <module>
  File "/tmp/tmp1bl0s_61/pip-7.1.2-py2.py3-none-any.whl/pip/_vendor/cachecontrol/cache.py", line 5, in <module>
  File "/tmp/cpython/Lib/threading.py", line 4, in <module>
    import _thread
ImportError: No module named '_thread'


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