Fix python-info-docstring-p

Fix python-info-docstring-p

* lisp/progmodes/python.el (python-info-docstring-p): Stop using
python-rx string-delimiter.

* test/lisp/progmodes/python-tests.el
(python-font-lock-escape-sequence-bytes-newline)
(python-font-lock-escape-sequence-hex-octal)
(python-font-lock-escape-sequence-unicode)
(python-font-lock-raw-escape-sequence): Mark as expected failures
until another bug in 'python-info-docstring-p' is corrected.
(python-info-docstring-p-7): New test.  (Bug#63622)