Add email.charset.SHORTEST, QP, BASE64 to the stub for Python 2 (#2857)

Also mark Python 3 constants as undocumented.

Fixes #2830

stdlib/2/email/charset.pyi

@@ -2,6 +2,10 @@ def add_charset(charset, header_enc=..., body_enc=..., output_charset=...) -> No
 def add_alias(alias, canonical) -> None: ...
 def add_codec(charset, codecname) -> None: ...
 
+QP: int  # undocumented
+BASE64: int  # undocumented
+SHORTEST: int  # undocumented
+
 class Charset:
     input_charset = ...
     header_encoding = ...