test__os.py: buffer/memoryview fix on PY3

thanks to @fantix for original patch.

greentest/test__os.py

+import sys
+import six
 from os import pipe
 from gevent import os
 from greentest import TestCase, main
@@ -26,12 +28,12 @@ class TestOS_tp(TestCase):
     def write(self, *args):
         return os.tp_write(*args)
 
-    def test_if_pipe_blocks(self):
+    def _test_if_pipe_blocks(self, buffer_class):
         r, w = self.pipe()
         # set nbytes such that for sure it is > maximum pipe buffer
         nbytes = 1000000
         block = 'x' * 4096
-        buf = buffer(block)
+        buf = buffer_class(block)
         # Lack of "nonlocal" keyword in Python 2.x:
         bytesread = [0]
         byteswritten = [0]
@@ -57,6 +59,16 @@ class TestOS_tp(TestCase):
         assert bytesread[0] == nbytes
         assert bytesread[0] == byteswritten[0]
 
+    if sys.version_info[0] < 3:
+
+        def test_if_pipe_blocks_buffer(self):
+            self._test_if_pipe_blocks(six.builtins.buffer)
+
+    if sys.version_info[:2] >= (2, 7):
+
+        def test_if_pipe_blocks_memoryview(self):
+            self._test_if_pipe_blocks(six.builtins.memoryview)
+
 
 if hasattr(os, 'make_nonblocking'):