Add undocumented methods codecs.utf_16_be_{decode,encode}. (#3091)

I found the signatures here:
https://github.com/python/cpython/blob/6a16b18224fa98f6d192aa5014affeccc0376eb3/Modules/_codecsmodule.c#L729

google/pytype#348 was opened against
pytype about utf_16_be_encode being missing.

Author: rchen152

stdlib/2and3/codecs.pyi

@@ -31,6 +31,8 @@ class _IncrementalDecoder(Protocol):
 def encode(obj: _Decoded, encoding: str = ..., errors: str = ...) -> _Encoded: ...
 def decode(obj: _Encoded, encoding: str = ..., errors: str = ...) -> _Decoded: ...
 def lookup(encoding: str) -> CodecInfo: ...
+def utf_16_be_decode(__obj: _Encoded, __errors: str = ..., __final: bool = ...) -> Tuple[_Decoded, int]: ...  # undocumented
+def utf_16_be_encode(__obj: _Decoded, __errors: str = ...) -> Tuple[_Encoded, int]: ...  # undocumented
 
 class CodecInfo(Tuple[_Encoder, _Decoder, _StreamReader, _StreamWriter]):
     @property