Issue #19543: Emit deprecation warning for known non-text encodings.

Backported issues #19619: encode() and decode() methods and constructors
of str, unicode and bytearray classes now emit deprecation warning for known
non-text encodings when Python is ran with the -3 option.

Backported issues #20404: io.TextIOWrapper (and hence io.open()) now uses the
internal codec marking system added to emit deprecation warning for known non-text
encodings at stream construction time when Python is ran with the -3 option.
diff --git a/Lib/encodings/rot_13.py b/Lib/encodings/rot_13.py
index 52b6431..4eaf433 100755
--- a/Lib/encodings/rot_13.py
+++ b/Lib/encodings/rot_13.py
@@ -44,6 +44,7 @@
         incrementaldecoder=IncrementalDecoder,
         streamwriter=StreamWriter,
         streamreader=StreamReader,
+        _is_text_encoding=False,
     )
 
 ### Decoding Map