Only use multiprocessing when the method is fork

In python3.8 on macos and in all versions on windows the multiprocessing
method is `spawn` which does not preserve class state.

src/flake8/checker.py

@@ -4,6 +4,7 @@ import errno
 import itertools
 import logging
 import signal
+import sys
 import tokenize
 from typing import Dict, List, Optional, Tuple
 
@@ -35,6 +36,18 @@ SERIAL_RETRY_ERRNOS = {
 }
 
 
+def _multiprocessing_is_fork():  # type () -> bool
+    """Class state is only preserved when using the `fork` strategy."""
+    if sys.version_info >= (3, 4):
+        return (
+            multiprocessing
+            # https://github.com/python/typeshed/pull/3415
+            and multiprocessing.get_start_method() == "fork"  # type: ignore
+        )
+    else:
+        return multiprocessing and not utils.is_windows()
+
+
 class Manager(object):
     """Manage the parallelism and checker instances for each plugin and file.
 
@@ -101,26 +114,13 @@ class Manager(object):
         # - we're processing a diff, which again does not work well with
         #   multiprocessing and which really shouldn't require multiprocessing
         # - the user provided some awful input
-        if not multiprocessing:
+        if not _multiprocessing_is_fork():
             LOG.warning(
                 "The multiprocessing module is not available. "
                 "Ignoring --jobs arguments."
             )
             return 0
 
-        if (
-            utils.is_windows()
-            and not utils.can_run_multiprocessing_on_windows()
-        ):
-            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."
-            )
-            return 0
-
         if utils.is_using_stdin(self.arguments):
             LOG.warning(
                 "The --jobs option is not compatible with supplying "

src/flake8/utils.py

@@ -298,30 +298,6 @@ 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:
-        bool
-    """
-    is_new_enough_python27 = (2, 7, 11) <= sys.version_info < (3, 0)
-    is_new_enough_python3 = sys.version_info > (3, 2)
-    return False and (is_new_enough_python27 or is_new_enough_python3)
-
-
 def is_using_stdin(paths):
     # type: (List[str]) -> bool
     """Determine if we're going to read from stdin.