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_peepholer.py

@@ -1,6 +1,6 @@
 import dis
 import sys
-from cStringIO import StringIO
+from io import StringIO
 import unittest
 
 def disassemble(func):