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_atexit.py

@@ -1,6 +1,6 @@
 import sys
 import unittest
-import StringIO
+import io
 import atexit
 from test import test_support
 
@@ -25,7 +25,7 @@ def raise2():
 
 class TestCase(unittest.TestCase):
     def setUp(self):
-        self.stream = StringIO.StringIO()
+        self.stream = io.StringIO()
         sys.stdout = sys.stderr = self.stream
         atexit._clear()