gh-83938, gh-122476: Stop incorrectly RFC 2047 encoding non-ASCII email addresses (#122540)

The email generators had been incorrectly flattening non-ASCII email
addresses to RFC 2047 encoded-word format, leaving them undeliverable.
(RFC 2047 prohibits use of encoded-word in an addr-spec.)
This change raises a HeaderWriteError when attempting to flatten an
EmailMessage with a non-ASCII addr-spec and a policy with utf8=False.
(Exception: If the non-ASCII address originated from parsing a message,
it will be flattened as originally parsed, without error.)  This also applies
to other contexts in which RFC2047 words are not allowed by the RFCs.

Non-ASCII email addresses are supported when using a policy with
utf8=True (such as email.policy.SMTPUTF8) under RFCs 6531 and 6532.

Non-ASCII email address domains (but not localparts) can also be used
with non-SMTPUTF8 policies by encoding the domain as an IDNA A-label.
(The email package does not perform this encoding, because it cannot
know whether the caller wants IDNA 2003, IDNA 2008, or some other
variant such as UTS #46.)

Co-authored-by: R. David Murray <rdmurray@bitdance.com>