SF patch# 1770008 by Christian Heimes (plus some extras).

Completely get rid of StringIO.py and cStringIO.c.

I had to fix a few tests and modules beyond what Christian did, and
invent a few conventions.  E.g. in elementtree, I chose to
write/return Unicode strings whe no encoding is given, but bytes when
an explicit encoding is given.  Also mimetools was made to always
assume binary files.

Lib/email/test/test_email_torture.py

@@ -9,7 +9,7 @@
 import sys
 import os
 import unittest
-from cStringIO import StringIO
+from io import StringIO
 from types import ListType
 
 from email.test.test_email import TestEmailBase