Changes in anticipation of stricter str vs. bytes enforcement.
diff --git a/Lib/test/test_io.py b/Lib/test/test_io.py
index f8a4938..656711f 100644
--- a/Lib/test/test_io.py
+++ b/Lib/test/test_io.py
@@ -305,7 +305,9 @@
 
 
 class BytesIOTest(MemorySeekTestMixin, unittest.TestCase):
-    buftype = bytes
+    @staticmethod
+    def buftype(s):
+        return s.encode("utf-8")
     ioclass = io.BytesIO
     EOF = b""