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.
diff --git a/Lib/test/test_csv.py b/Lib/test/test_csv.py
index 6098285..21cc408 100644
--- a/Lib/test/test_csv.py
+++ b/Lib/test/test_csv.py
@@ -5,7 +5,7 @@
 import sys
 import os
 import unittest
-from StringIO import StringIO
+from io import StringIO
 from tempfile import TemporaryFile
 import csv
 import gc