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/cookielib.py

@@ -58,7 +58,7 @@ def _warn_unhandled_exception():
     # catching input that's bad in unexpected ways.  Warn if any
     # exceptions are caught there.
     import warnings, traceback, StringIO
-    f = StringIO.StringIO()
+    f = io.StringIO()
     traceback.print_exc(None, f)
     msg = f.getvalue()
     warnings.warn("cookielib bug!\n%s" % msg, stacklevel=2)