Check whether running on Windows Subsystem for Linux. (dask#2543)

This works around an issue in WSL that causes Distributed with
default config to crash on initializion. Fix by checking whether
the stack is WSL. If so, proceed as if Windows is running, which
fixes the issue.

distributed/utils.py

@@ -13,6 +13,7 @@
 from numbers import Number
 import operator
 import os
+import platform
 import re
 import shutil
 import socket
@@ -62,7 +63,12 @@
 
 
 def _initialize_mp_context():
-    if PY3 and not sys.platform.startswith("win") and "PyPy" not in sys.version:
+    if (
+        PY3
+        and not sys.platform.startswith("win")
+        and "PyPy" not in sys.version
+        and "microsoft" not in platform.uname()[3].lower()
+    ):
         method = dask.config.get("distributed.worker.multiprocessing-method")
         ctx = multiprocessing.get_context(method)
         # Makes the test suite much faster