Deprecate CUnicodeIO

I don't see any usage of it, but it's easy enough to deprecate it for a
cycle just in case.

Closes gh-10140

IPython/lib/pretty.py

@@ -84,6 +84,7 b' import re'
 import datetime
 from collections import deque
 from io import StringIO
+from warnings import warn
 
 from IPython.utils.py3compat import PYPY, cast_unicode
 from IPython.utils.encoding import get_stream_enc
@@ -107,7 +108,12 b' def _safe_getattr(obj, attr, default=None):'
     except Exception:
         return default
 
-CUnicodeIO = StringIO
+class CUnicodeIO(StringIO):
+    def __init__(self, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+        warn(("CUnicodeIO is deprecated since IPython 6.0. "
+              "Please use io.StringIO instead."),
+             DeprecationWarning, stacklevel=2)
 
 def pretty(obj, verbose=False, max_width=79, newline='\n', max_seq_length=MAX_SEQ_LENGTH):
     """