tox: use version ranges #113

Job	Run time
Verify dev env (ubuntu-latest)	1s
Verify dev env (windows-latest)	34s
Build & verify package	1s
Ensure 100% test coverage	1d 0h 0m 1s
Run doctests	0s
Tests & Mypy API on ${{ matrix.python-version }}	0s
Check types using supported type checkers	0s
Visual check for color settings using env variables	0s
Ensure everything required is passing for branch protection	5s
	1d 0h 0m 42s

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