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tox: add Python 3.11, 3.12, 3.13 to test matrix #114

Job	Run time
Packaging	4s
Black (linter)	3s
Workflow information	3s
Flake8 (linter)	3s
Build documentation	3s
Mypy (static type checker)	3s
Unit Tests using Python ${{ matrix.python }} on Ubuntu	-1s
	18s

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