tree-sitter compatibility

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python-ts-mode seems to work OK with the highlighting, but docstring filling is badly broken

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I think that this will be waaaaaay easier than the shenanigans we are doing with syntax-ppss; we should be able to just do something like

(defun pdm-bracket-docstring ()
  (let* ((the-node (treesit-node-at (point)))
         (start (treesit-node-start the-node))
         (end (treesit-node-end the-node)))
    (cons start end)
    ))