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/util.py b/Lib/wsgiref/util.py
index 5b44eda..a4ca02f 100644
--- a/Lib/wsgiref/util.py
+++ b/Lib/wsgiref/util.py
@@ -149,7 +149,7 @@
     environ.setdefault('wsgi.multithread', 0)
     environ.setdefault('wsgi.multiprocess', 0)
 
-    from StringIO import StringIO
+    from io import StringIO
     environ.setdefault('wsgi.input', StringIO(""))
     environ.setdefault('wsgi.errors', StringIO())
     environ.setdefault('wsgi.url_scheme',guess_scheme(environ))