Make syntax-ppss more accurate for Python triple quotes (bug#49518)

Make syntax-ppss more accurate for Python triple quotes (bug#49518)

By putting delimiter syntax on the "inside" of Python triple-quoted
strings, this makes syntax-ppss be more accurate and thus helps things
like electric-pair-mode.  Also, the test
python-syntax-after-python-backspace now passes, again.

* lisp/progmodes/python.el (python-syntax-stringify): Put
delimiter syntax in "inner" of the surrouding triple quotes.

* test/lisp/progmodes/python-tests.el
(python-syntax-after-python-backspace): Passes again.