Fix singledispatch for asyncio on py26.

Disable singledispatch tests for twisted on py26 because it doesn't
work.

setup.py

@@ -122,6 +122,9 @@ def build_extension(self, ext):
 if setuptools is not None:
     # If setuptools is not available, you're on your own for dependencies.
     install_requires = []
+    if sys.version_info < (2, 7):
+        # Only needed indirectly, for singledispatch.
+        install_requires.append('ordereddict')
     if sys.version_info < (3, 2):
         install_requires.append('backports.ssl_match_hostname')
     if sys.version_info < (3, 4):

tornado/test/twisted_test.py

@@ -72,6 +72,9 @@
 skipIfNoTwisted = unittest.skipUnless(have_twisted,
                                       "twisted module not present")
 
+skipIfPy26 = unittest.skipIf(sys.version_info < (2, 7),
+                             "twisted incompatible with singledispatch in py26")
+
 
 def save_signal_handlers():
     saved = {}
@@ -492,6 +495,7 @@ def testTornadoServerTwistedClientReactor(self):
             'http://127.0.0.1:%d' % self.tornado_port, self.run_reactor)
         self.assertEqual(response, 'Hello from tornado!')
 
+    @skipIfPy26
     def testTornadoServerTwistedCoroutineClientIOLoop(self):
         self.start_tornado_server()
         response = self.twisted_coroutine_fetch(
@@ -500,6 +504,7 @@ def testTornadoServerTwistedCoroutineClientIOLoop(self):
 
 
 @skipIfNoTwisted
+@skipIfPy26
 class ConvertDeferredTest(unittest.TestCase):
     def test_success(self):
         @inlineCallbacks