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/test/test_deque.py

@@ -4,7 +4,7 @@ from test import test_support, seq_tests
 from weakref import proxy
 import copy
 import pickle
-from cStringIO import StringIO
+from io import StringIO
 import random
 import os