gh-67022: Document bytes/str inconsistency in email.header.decode_header() and suggest email.headerregistry.HeaderRegistry as a sane alternative (#92900)

* gh-67022: Document bytes/str inconsistency in email.header.decode_header()

This function's possible return types have been surprising and error-prone
for the entirety of its Python 3.x history. It can return either:

1. `typing.List[typing.Tuple[bytes, typing.Optional[str]]]` of length >1
2. or `typing.List[typing.Tuple[str, None]]`, of length exactly 1

This means that any user of this function must be prepared to accept either
`bytes` or `str` for the first member of the 2-tuples it returns, which is a
very surprising behavior in Python 3.x, particularly given that the second
member of the tuple is supposed to represent the charset/encoding of the
first member.

This patch documents the behavior of this function, and adds test cases
to demonstrate it.

As discussed in bpo-22833, this cannot be changed in a backwards-compatible
way, and some users of this function depend precisely on the existing
behavior.

Add warnings about obsolescence of 'email.header.decode_header' and 'email.header.make_header' functions.

Recommend use of `email.headerregistry.HeaderRegistry` instead, as suggested
in https://github.com/python/cpython/pull/92900#discussion_r1112472177

Lib/email/header.py

@@ -59,16 +59,22 @@ _max_append = email.quoprimime._max_append
 def decode_header(header):
     """Decode a message header value without converting charset.
 
-    Returns a list of (string, charset) pairs containing each of the decoded
-    parts of the header.  Charset is None for non-encoded parts of the header,
-    otherwise a lower-case string containing the name of the character set
-    specified in the encoded string.
+    For historical reasons, this function may return either:
+
+    1. A list of length 1 containing a pair (str, None).
+    2. A list of (bytes, charset) pairs containing each of the decoded
+       parts of the header.  Charset is None for non-encoded parts of the header,
+       otherwise a lower-case string containing the name of the character set
+       specified in the encoded string.
 
     header may be a string that may or may not contain RFC2047 encoded words,
     or it may be a Header object.
 
     An email.errors.HeaderParseError may be raised when certain decoding error
     occurs (e.g. a base64 decoding exception).
+
+    This function exists for backwards compatibility only. For new code, we
+    recommend using email.headerregistry.HeaderRegistry instead.
     """
     # If it is a Header object, we can just return the encoded chunks.
     if hasattr(header, '_chunks'):
@@ -161,6 +167,9 @@ def make_header(decoded_seq, maxlinelen=None, header_name=None,
     This function takes one of those sequence of pairs and returns a Header
     instance.  Optional maxlinelen, header_name, and continuation_ws are as in
     the Header constructor.
+
+    This function exists for backwards compatibility only, and is not
+    recommended for use in new code.
     """
     h = Header(maxlinelen=maxlinelen, header_name=header_name,
                continuation_ws=continuation_ws)