test_venv should skip test_with_pip() test when the zlib module is missing.

x86-64 Sierra 3.6 failure:
http://buildbot.python.org/all/#/builders/20/builds/62

test_with_pip (test.test_venv.EnsurePipTest) ... FAIL

======================================================================
FAIL: test_with_pip (test.test_venv.EnsurePipTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/buildbot/buildarea/3.6.billenstein-sierra/build/Lib/test/test_venv.py", line 368, in do_test_with_pip
    with_pip=True)
  File "/Users/buildbot/buildarea/3.6.billenstein-sierra/build/Lib/test/test_venv.py", line 61, in run_with_capture
    func(*args, **kwargs)
subprocess.CalledProcessError: Command '['/private/var/folders/sy/9hwmqyx14s11577cvgzwf2v40000gt/T/tmpzg_lamk1/bin/python.exe', '-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 "/Users/buildbot/buildarea/3.6.billenstein-sierra/build/Lib/test/test_venv.py", line 428, in test_with_pip
    self.do_test_with_pip(False)
  File "/Users/buildbot/buildarea/3.6.billenstein-sierra/build/Lib/test/test_venv.py", line 374, in do_test_with_pip
    self.fail(msg.format(exc, details))
AssertionError: Command '['/private/var/folders/sy/9hwmqyx14s11577cvgzwf2v40000gt/T/tmpzg_lamk1/bin/python.exe', '-Im', 'ensurepip', '--upgrade', '--default-pip']' returned non-zero exit status 1.

**Subprocess Output**
Traceback (most recent call last):
  File "/Users/buildbot/buildarea/3.6.billenstein-sierra/build/Lib/runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "/Users/buildbot/buildarea/3.6.billenstein-sierra/build/Lib/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/Users/buildbot/buildarea/3.6.billenstein-sierra/build/Lib/ensurepip/__main__.py", line 5, in <module>
    sys.exit(ensurepip._main())
  File "/Users/buildbot/buildarea/3.6.billenstein-sierra/build/Lib/ensurepip/__init__.py", line 204, in _main
    default_pip=args.default_pip,
  File "/Users/buildbot/buildarea/3.6.billenstein-sierra/build/Lib/ensurepip/__init__.py", line 117, in _bootstrap
    return _run_pip(args + [p[0] for p in _PROJECTS], additional_paths)
  File "/Users/buildbot/buildarea/3.6.billenstein-sierra/build/Lib/ensurepip/__init__.py", line 27, in _run_pip
    import pip
zipimport.ZipImportError: can't decompress data; zlib not available

Oh, Serhiy already fixed it: bpo-31999.