[3.15] gh-103925: Fix csv.Sniffer for a quoted field ending a CRLF line (GH-103926...

[3.15] gh-103925: Fix csv.Sniffer for a quoted field ending a CRLF line (GH-103926) (GH-154322)

"$" does not match before "\r" even in the MULTILINE mode, so such a
field was not found and the delimiter was guessed from character
frequencies instead, which could give a letter.
(cherry picked from commit 70f7c6c0f2ddfd3b447946f1b926776b2a344703)

Co-authored-by: Zhou Wei <lilaboc.cn@gmail.com>
Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>