initial support for tree-sitter by honoring python-base-mode

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Fixes #28

I haven't really tested this yet, but it seems like a necessary first step.

[anachronic]

I know I haven't been really attemptive of any solutions in this repo in a long time. I'll check it as soon as I can. I do know where to start.

Just wanted to say thank you, and I'll be trying this out

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Thank you!

I know it's tough to keep up with maintenance on this stuff, and it's volunteer effort.

[lhernanz]

I think that it should be enough to replace the 3 references to derived-mode-p 'python-mode to derived-mode-p 'python-base-mode. At least that change is working for me.