Fix string fontification for tree-sitter python-mode

Fix string fontification for tree-sitter python-mode

Now when user inserts a ending quote, the whole string is ganranteed
to be refontified and redisplayed.

* lisp/progmodes/python.el (python--treesit-fontify-string): Change
docstring, now it's called on the leading quote.
(python--treesit-fontify-string-end): New function.
(python--treesit-settings): Capture both leading and ending quote in a
string.