[python] Validate pyproject.toml and fix the pyproject.toml version constraint format

[4c0n]

Fixes: #21335

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[4c0n]

Does anyone have any clues as to why mypy is suddenly failing? This does not seem to correlate with the changes this PR is making...

[wing328]

agreed it's not related to this PR. i'll take a look

[4c0n]

@wing328 Something like this should fix the issue: #21423 it's not very pretty, but it's probably better than just excluding those lines, which would be another option (using # type: ignore).

[wing328]

thanks @4c0n. just merged your fix