decode_header

Decode a message header value without converting charset.

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.