-n auto and tox on windows does not work

multiprocessing.cpu_count() uses int(os.environ['NUMBER_OF_PROCESSORS']) to get the number of CPUs. I guess this does not work as tox cleans all the environment variables. Thus I suggest using another approach. There are several listed in http://stackoverflow.com/questions/1006289/how-to-find-out-the-number-of-cpus-using-python.

File "C:\Python27\Lib\runpy.py", line 162, in _run_module_as_main
    "__main__", fname, loader, pkg_name)
  File "C:\Python27\Lib\runpy.py", line 72, in _run_code
    exec code in run_globals
  File "c:\windows\temp\devpi-test829\ngc-tsg-1.17.0.3687097\.tox\python\Scripts\py.test.EXE\__main__.py", line 9, in <module>
  File "c:\windows\temp\devpi-test829\ngc-tsg-1.17.0.3687097\.tox\python\lib\site-packages\_pytest\config.py", line 39, in main
    config = _prepareconfig(args, plugins)
  File "c:\windows\temp\devpi-test829\ngc-tsg-1.17.0.3687097\.tox\python\lib\site-packages\_pytest\config.py", line 118, in _prepareconfig
    pluginmanager=pluginmanager, args=args)
  File "c:\windows\temp\devpi-test829\ngc-tsg-1.17.0.3687097\.tox\python\lib\site-packages\_pytest\vendored_packages\pluggy.py", line 724, in __call__
    return self._hookexec(self, self._nonwrappers + self._wrappers, kwargs)
  File "c:\windows\temp\devpi-test829\ngc-tsg-1.17.0.3687097\.tox\python\lib\site-packages\_pytest\vendored_packages\pluggy.py", line 338, in _hookexec
    return self._inner_hookexec(hook, methods, kwargs)
  File "c:\windows\temp\devpi-test829\ngc-tsg-1.17.0.3687097\.tox\python\lib\site-packages\_pytest\vendored_packages\pluggy.py", line 333, in <lambda>
    _MultiCall(methods, kwargs, hook.spec_opts).execute()
  File "c:\windows\temp\devpi-test829\ngc-tsg-1.17.0.3687097\.tox\python\lib\site-packages\_pytest\vendored_packages\pluggy.py", line 595, in execute
    return _wrapped_call(hook_impl.function(*args), self.execute)
  File "c:\windows\temp\devpi-test829\ngc-tsg-1.17.0.3687097\.tox\python\lib\site-packages\_pytest\vendored_packages\pluggy.py", line 249, in _wrapped_call
    wrap_controller.send(call_outcome)
  File "c:\windows\temp\devpi-test829\ngc-tsg-1.17.0.3687097\.tox\python\lib\site-packages\_pytest\helpconfig.py", line 28, in pytest_cmdline_parse
    config = outcome.get_result()
  File "c:\windows\temp\devpi-test829\ngc-tsg-1.17.0.3687097\.tox\python\lib\site-packages\_pytest\vendored_packages\pluggy.py", line 279, in get_result
    _reraise(*ex)  # noqa
  File "c:\windows\temp\devpi-test829\ngc-tsg-1.17.0.3687097\.tox\python\lib\site-packages\_pytest\vendored_packages\pluggy.py", line 264, in __init__
    self.result = func()
  File "c:\windows\temp\devpi-test829\ngc-tsg-1.17.0.3687097\.tox\python\lib\site-packages\_pytest\vendored_packages\pluggy.py", line 596, in execute
    res = hook_impl.function(*args)
  File "c:\windows\temp\devpi-test829\ngc-tsg-1.17.0.3687097\.tox\python\lib\site-packages\_pytest\config.py", line 856, in pytest_cmdline_parse
    self.parse(args)
  File "c:\windows\temp\devpi-test829\ngc-tsg-1.17.0.3687097\.tox\python\lib\site-packages\_pytest\config.py", line 961, in parse
    self._preparse(args, addopts=addopts)
  File "c:\windows\temp\devpi-test829\ngc-tsg-1.17.0.3687097\.tox\python\lib\site-packages\_pytest\config.py", line 926, in _preparse
    self.known_args_namespace = ns = self._parser.parse_known_args(args, namespace=self.option.copy())
  File "c:\windows\temp\devpi-test829\ngc-tsg-1.17.0.3687097\.tox\python\lib\site-packages\_pytest\config.py", line 491, in parse_known_args
    return self.parse_known_and_unknown_args(args, namespace=namespace)[0]
  File "c:\windows\temp\devpi-test829\ngc-tsg-1.17.0.3687097\.tox\python\lib\site-packages\_pytest\config.py", line 499, in parse_known_and_unknown_args
    return optparser.parse_known_args(args, namespace=namespace)
  File "C:\Python27\Lib\argparse.py", line 1720, in parse_known_args
    namespace, args = self._parse_known_args(args, namespace)
  File "C:\Python27\Lib\argparse.py", line 1926, in _parse_known_args
    start_index = consume_optional(start_index)
  File "C:\Python27\Lib\argparse.py", line 1866, in consume_optional
    take_action(action, args, option_string)
  File "C:\Python27\Lib\argparse.py", line 1778, in take_action
    argument_values = self._get_values(action, argument_strings)
  File "C:\Python27\Lib\argparse.py", line 2204, in _get_values
    value = self._get_value(action, arg_string)
  File "C:\Python27\Lib\argparse.py", line 2233, in _get_value
    result = type_func(arg_string)
  File "c:\windows\temp\devpi-test829\ngc-tsg-1.17.0.3687097\.tox\python\lib\site-packages\xdist\plugin.py", line 8, in parse_numprocesses
    return multiprocessing.cpu_count()
  File "C:\Python27\Lib\multiprocessing\__init__.py", line 136, in cpu_count
    raise NotImplementedError('cannot determine number of cpus')
NotImplementedError: cannot determine number of cpus

Start in xdist/plugin.py at parse_numprocesses and compare its multiprocessing.cpu_count() call with the Windows traceback showing failure under tox. Reproduce the -n auto case on Windows with tox, then verify that automatic process selection no longer raises NotImplementedError.