Stackless issue python#98: Skip the test case, if stackless is compiled without threads.

The test case needs threding.

https://bitbucket.org/stackless-dev/stackless/issues/98
(grafted from 16c5769adf70810e23d480a51dddc60f34384251)

Author: Anselm Kruis

Stackless/unittests/test_defects.py

@@ -493,6 +493,7 @@ def other_thread(self, pickler, c):
         else:
             self.killed = False
 
+    @unittest.skipUnless(withThreads, "requires thread support")
     def test_kill_during_cPickle_stack_switch(self):
         # this test kills the main/current tasklet of a other-thread,
         # which is fast-pickling a recursive structure. This leads to an