minify: avoid cStringIO use.

On 2.7 it was "accidentally fine" because the buffer object the StringIO
was initialized from happened to look like ASCII, but in 2.6 either
UCS-2 or UCS-4 is used for that buffer, and so the result was junk.

Just use the io module everywhere if we can, falling back to pure-Python
StringIO for Python<2.6.

mitogen/minify.py

@@ -29,9 +29,9 @@
 import sys
 
 try:
-    from cStringIO import StringIO
-except ImportError:
     from io import StringIO
+except ImportError:
+    from StringIO import StringIO
 
 import mitogen.core