Tree-sitter integration in python.el

[Yuan Fu]

Hi, 

I’ve added tree-sitter version for font-lock and which-func in python.el. And 
I’d love to hear some feedback from python.el maintainers. Specifically, does 
it look right and which other part of python.el could actually benefit from a 
parse tree? I wrote a tree-sitter imenu indexer for python and it performed 
worse than the current one, presumably because it traverses the whole parse 
tree whereas the current one only scans the buffer once or so and do some regex 
matching.

Here is the commit: 
https://git.savannah.gnu.org/cgit/emacs.git/commit/?h=feature/tree-sitter&id=1cdb24fe35a9ff2e4f92c5acc93a5a5b0e70d93f

Yuan