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_rfc822.py b/Lib/test/test_rfc822.py
index c355950..b563c9c 100644
--- a/Lib/test/test_rfc822.py
+++ b/Lib/test/test_rfc822.py
@@ -4,9 +4,9 @@
 from test import test_support
 
 try:
-    from cStringIO import StringIO
+    from io import StringIO
 except ImportError:
-    from StringIO import StringIO
+    from io import StringIO
 
 
 class MessageTestCase(unittest.TestCase):