Fix mutable default argument in multiprocessing.dummy.DummyProcess

Change the default value of the 'kwargs' parameter in
DummyProcess.__init__() from {} to None to avoid mutable default
argument issues. This makes DummyProcess consistent with
threading.Thread and threading.Timer which already use this pattern.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Reported by: Dmitrii Chuprov <cheese@altlinux.org>

Signed-off-by: Denis Sergeev <zeff@altlinux.org>

Author: zeff-ir

Lib/multiprocessing/dummy/__init__.py

@@ -33,7 +33,9 @@
 
 class DummyProcess(threading.Thread):
 
-    def __init__(self, group=None, target=None, name=None, args=(), kwargs={}):
+    def __init__(self, group=None, target=None, name=None, args=(), kwargs=None):
+        if kwargs is None:
+            kwargs = {}
         threading.Thread.__init__(self, group, target, name, args, kwargs)
         self._pid = None
         self._children = weakref.WeakKeyDictionary()