call site 3 for execnet.PopenGateway.__init__
execnet/testing/test_gateway.py - line 514
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   def test_threads_twice():
->     gw = py.execnet.PopenGateway()
       gw.remote_init_threads(3)
       py.test.raises(IOError, gw.remote_init_threads, 3)
       gw.exit()