bpo-40597: email: Use CTE if lines are longer than max_line_length consistently (gh-20038)

raw_data_manager (default for EmailPolicy, EmailMessage)
does correct wrapping of 'text' parts as long as the message contains
characters outside of 7bit US-ASCII set: base64 or qp
Content-Transfer-Encoding is applied if the lines would be too long
without it.  It did not, however, do this for ascii-only text,
which could result in lines that were longer than
policy.max_line_length or even the rfc 998  maximum.

This changeset fixes the heuristic so that if lines are longer than
policy.max_line_length, it will always apply a
content-transfer-encoding so that the lines are wrapped correctly.