Improve Python typing in dsl plugin

[ratijas]

Thanks to a gentle and encouraging ping at this thread, I decided to take a look at how my old rusty code is doin'.

Needless to say, there's a lot of room for improvements — considering a jump from Python 2.7 at that time to 3.9+ nowadays.

Also, @ilius are you sure those self: "typing.Self" are really worth putting everywhere? I don't feel like they bring much value, and they cost a lot of visual noise. Actually, I've never seen these kind of hints.

[ilius]

I don't like adding typing.Self either, but I added them to get rid of errors given by ruff tool which I think is the only fast type checker for Python. I can't imagine running mypy on every commit, it's so slow.

[ilius]

Thanks for PR.

If you have time, please take a look at this Go program I wrote:
https://github.com/ilius/repassgen

Specially Go files starting with lex and functions starting with Lex.

I think this is the perfect design for things like template rendering, and might fit perfectly for DSL -> HTML transformation as well (since we don't need to extract the parse tree). It can make the code much nicer and easier to maintain.