Fix lint tests by explicitly adding Python 3.13 to GitHub and tox.

[fschulze]

The mypy tests start failing with mypy 1.11.0 and seem to be related to the ast deprecations. I'll look into that next.

[coveralls]

Pull Request Test Coverage Report for Build 12407619985

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 92.354%

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Totals
Change from base Build 12280815714:	0.0%
Covered Lines:	3907
Relevant Lines:	4157

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💛 - Coveralls

[malthe]

@fschulze thanks, I have been a bit swamped lately, but was going to have a look eventually at those ast deprecations. Should be relatively simple to fix, just a bit of a chore.