Switch wsgiref to io.{StringIO,BytesIO}.  This shuts up the test failures.
(I'm not sure about it actually working though.)
diff --git a/Lib/wsgiref/simple_server.py b/Lib/wsgiref/simple_server.py
index abb3620..980f97a 100644
--- a/Lib/wsgiref/simple_server.py
+++ b/Lib/wsgiref/simple_server.py
@@ -163,7 +163,7 @@
 
 
 def demo_app(environ,start_response):
-    from StringIO import StringIO
+    from io import StringIO
     stdout = StringIO()
     print("Hello world!", file=stdout)
     print(file=stdout)