bug#67703: Fix python-ts-mode syntax higlighting as to account for default arguments

[Eli Zaretskii]

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Forwarding to the bug tracker:

> From: Maciej Kalandyk <m.kalandyk@outlook.com>
> Date: Sun, 10 Dec 2023 00:57:15 +0000
> msip_labels:
> 
> in this code without patch
> 
> def test(x=None):
>       return x
> 
> x=None in function signature won't be highlighted as definition