[3.14] gh-145831: email.quoprimime: `decode()` leaves stray `\r` when `eol='\r\n'` (GH-145832) (#148312)

decoded[:-1] only strips one character, leaving a stray \r when eol
is two characters. Fix: decoded[:-len(eol)].
(cherry picked from commit 1a0edb1fa899c067f19b09598b45cdb6e733c4ee)

Co-authored-by: Stefan Zetzsche <120379523+stefanzetzsche@users.noreply.github.com>