FIX ApiModule can be vendored in the pytest package (#135)

Author: ogrisel

tests/cloudpickle_test.py

@@ -690,10 +690,10 @@ def f():
             s = py.builtin.set([1])
             return s.pop()
 
-        # some setup is required to allow pytest apimodules to be correctly serializable.
+        # some setup is required to allow pytest apimodules to be correctly
+        # serializable.
         from cloudpickle import CloudPickler
-        from py._apipkg import ApiModule
-        CloudPickler.dispatch[ApiModule] = CloudPickler.save_module
+        CloudPickler.dispatch[type(py.builtin)] = CloudPickler.save_module
         g = cloudpickle.loads(cloudpickle.dumps(f))
 
         result = g()