Explicitly prevent Python 3.12

[maresb]

To discuss: If we don't support 3.12, maybe we should enforce this here in pyproject.toml?

I'm not convinced that this is the right move since it might slightly hinder testing. But if we are explicit, then this seems to me like the correct place for it.

Motivation for these changes

Source: #441 (comment)

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Maintenance

• Put explicit Python version upper-bound

[ricardoV94]

Should be enough to do it in PyTensor, since there's where the restriction comes from?

[maresb]

What do you mean by "do it in PyTensor"? This is the pyproject.toml for PyTensor, right?

[ricardoV94]

I don't know why I thought this was the PyMC repo. Sorry :)