Diable multiprocessing behaviour on Windows

Due to https://bugs.python.org/issue27649, we cannot continue to
expect multiprocessing to work as we expect and document it on Windows.
As such, we are going to revert back to our previous behaviour of
disabling it across all versions of Python on Windows to provide the
default expected behaviour of Flake8 on that Operating System.

docs/source/release-notes/3.0.3.rst

@@ -1,6 +1,9 @@
 3.0.3 -- 2016-07-28 (unreleased)
 --------------------------------
 
+- Disable ``--jobs`` for any version of Python on Windows.
+  (See also `this Python bug report`_)
+
 - Fix ``# noqa`` comments followed by a ``:`` and explanation broken by
   3.0.0 (See also `GitLab#178`_)
 
@@ -12,3 +15,6 @@
     https://gitlab.com/pycqa/flake8/issues/178
 .. _GitLab#195:
     https://gitlab.com/pycqa/flake8/issues/195
+
+.. _this Python bug report:
+    https://bugs.python.org/issue27649

src/flake8/checker.py

@@ -167,7 +167,8 @@ class Manager(object):
 
         if (utils.is_windows() and
                 not utils.can_run_multiprocessing_on_windows()):
-            LOG.warning('The --jobs option is only available on Windows on '
+            LOG.warning('The --jobs option is not available on Windows due to'
+                        ' a bug (https://bugs.python.org/issue27649) in '
                         'Python 2.7.11+ and 3.3+. We have detected that you '
                         'are running an unsupported version of Python on '
                         'Windows. Ignoring --jobs arguments.')

src/flake8/utils.py

@@ -164,10 +164,20 @@ def is_windows():
     return os.name == 'nt'
 
 
+# NOTE(sigmavirus24): If and when https://bugs.python.org/issue27649 is fixed,
+# re-enable multiprocessing support on Windows.
 def can_run_multiprocessing_on_windows():
     # type: () -> bool
     """Determine if we can use multiprocessing on Windows.
 
+    This presently will **always** return False due to a `bug`_ in the
+    :mod:`multiprocessing` module on Windows. Once fixed, we will check
+    to ensure that the version of Python contains that fix (via version
+    inspection) and *conditionally* re-enable support on Windows.
+
+    .. _bug:
+        https://bugs.python.org/issue27649
+
     :returns:
         True if the version of Python is modern enough, otherwise False
     :rtype:
@@ -175,7 +185,7 @@ def can_run_multiprocessing_on_windows():
     """
     is_new_enough_python27 = (2, 7, 11) <= sys.version_info < (3, 0)
     is_new_enough_python3 = sys.version_info > (3, 2)
-    return is_new_enough_python27 or is_new_enough_python3
+    return False and (is_new_enough_python27 or is_new_enough_python3)
 
 
 def is_using_stdin(paths):

tests/unit/test_checker_manager.py

@@ -25,7 +25,8 @@ def test_oserrors_cause_serial_fall_back():
     assert manager.using_multiprocessing is False
 
 
-def test_oserrors_are_reraised():
+@mock.patch('flake8.utils.is_windows', return_value=False)
+def test_oserrors_are_reraised(is_windows):
     """Verify that OSErrors will cause the Manager to fallback to serial."""
     err = OSError(errno.EAGAIN, 'Ominous message')
     style_guide = style_guide_mock()