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

Doc/library/email.header.rst

@@ -178,16 +178,36 @@ The :mod:`email.header` module also provides the following convenient functions.
    Decode a message header value without converting the character set. The header
    value is in *header*.
 
-   This function returns a list of ``(decoded_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:
 
-   Here's an example::
+   1. A list of pairs containing each of the decoded parts of the header,
+      ``(decoded_bytes, charset)``, where *decoded_bytes* is always an instance of
+      :class:`bytes`, and *charset* is either:
+
+        - A lower case string containing the name of the character set specified.
+
+        - ``None`` for non-encoded parts of the header.
+
+   2. A list of length 1 containing a pair ``(string, None)``, where
+      *string* is always an instance of :class:`str`.
+
+   An :exc:`email.errors.HeaderParseError` may be raised when certain decoding
+   errors occur (e.g. a base64 decoding exception).
+
+   Here are examples:
 
       >>> from email.header import decode_header
       >>> decode_header('=?iso-8859-1?q?p=F6stal?=')
       [(b'p\xf6stal', 'iso-8859-1')]
+      >>> decode_header('unencoded_string')
+      [('unencoded_string', None)]
+      >>> decode_header('bar =?utf-8?B?ZsOzbw==?=')
+      [(b'bar ', None), (b'f\xc3\xb3o', 'utf-8')]
+
+   .. note::
+
+       This function exists for for backwards compatibility only. For
+       new code, we recommend using :class:`email.headerregistry.HeaderRegistry`.
 
 
 .. function:: make_header(decoded_seq, maxlinelen=None, header_name=None, continuation_ws=' ')
@@ -203,3 +223,7 @@ The :mod:`email.header` module also provides the following convenient functions.
    :class:`Header` instance.  Optional *maxlinelen*, *header_name*, and
    *continuation_ws* are as in the :class:`Header` constructor.
 
+   .. note::
+
+       This function exists for for backwards compatibility only, and is
+       not recommended for use in new code.