Fix and deprecated the unicode_internal codec

unicode_internal codec uses Py_UNICODE instead of the real internal
representation (PEP 393: Py_UCS1, Py_UCS2 or Py_UCS4) for backward
compatibility.
diff --git a/Doc/whatsnew/3.3.rst b/Doc/whatsnew/3.3.rst
index 911d8d9..7f4517f 100644
--- a/Doc/whatsnew/3.3.rst
+++ b/Doc/whatsnew/3.3.rst
@@ -250,6 +250,8 @@
 
 (:issue:`12100`)
 
+The ``unicode_internal`` codec has been deprecated.
+
 crypt
 -----