bug#55702: 29.0.50; python-mark-defun does not handle backslash escaped newline

[kobarity]

When the following steps are performed, the first line "def \" will
not be included in the region.

1. emacs -Q
2. Load the following Python file using M-x find-file

#+begin_src python
def \
        foo(x):
    return x
#+end_src

3. M-x forward-line (Move point to the line "foo(x):".)
4. M-x python-mark-defun

In GNU Emacs 29.0.50 (build 4, x86_64-pc-linux-gnu)
 of 2022-05-21 built on ubuntu
Repository revision: 208102fa470e3417320062cdb48a9967d80bf092
Repository branch: master
System Description: Ubuntu 22.04 LTS

Configured using:
 'configure --without-x --with-gnutls=ifavailable'

Configured features:
ACL LIBXML2 MODULES NOTIFY INOTIFY PDUMPER SECCOMP SOUND SQLITE3 THREADS
XIM ZLIB

Important settings:
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  show-paren-mode: t
  electric-indent-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  line-number-mode: t
  indent-tabs-mode: t
  transient-mark-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t

Load-path shadows:
None found.

Features:
(shadow regexp-opt sort mail-extr face-remap help-mode tool-bar emacsbug
message mailcap yank-media rmc puny dired dired-loaddefs rfc822 mml
mml-sec password-cache epa derived epg rfc6068 epg-config gnus-util
text-property-search time-date seq mm-decode mm-bodies mm-encode
mail-parse rfc2231 mailabbrev gmm-utils mailheader cl-loaddefs cl-lib
sendmail rfc2047 rfc2045 ietf-drums mm-util mail-prsvr mail-utils
term/screen term/xterm xterm gv subr-x byte-opt bytecomp byte-compile
cconv iso-transl tooltip eldoc paren electric uniquify ediff-hook
vc-hooks lisp-float-type elisp-mode tabulated-list replace newcomment
text-mode lisp-mode prog-mode register page tab-bar menu-bar rfn-eshadow
isearch easymenu timer select mouse jit-lock font-lock syntax font-core
term/tty-colors frame minibuffer nadvice simple cl-generic cham georgian
utf-8-lang misc-lang vietnamese tibetan thai tai-viet lao korean
japanese eucjp-ms cp51932 hebrew greek romanian slovak czech european
ethiopic indian cyrillic chinese composite emoji-zwj charscript charprop
case-table epa-hook jka-cmpr-hook help abbrev obarray oclosure
cl-preloaded button loaddefs faces cus-face macroexp files window
text-properties overlay sha1 md5 base64 format env code-pages mule
custom widget keymap hashtable-print-readable backquote threads inotify
multi-tty make-network-process emacs)

Memory information:
((conses 16 48401 6401)
 (symbols 48 5803 1)
 (strings 32 16090 1890)
 (string-bytes 1 510153)
 (vectors 16 9643)
 (vector-slots 8 112266 9121)
 (floats 8 25 264)
 (intervals 56 385 66)
 (buffers 992 12))