tox: Add support for tox 4.x
Some deprecated tox 3.x features have been removed in tox 4.x and some install behaviors changed between the two versions.
So add tox 4.x support while keeping tox >= 3.7 compatibility.
Some deprecated tox 3.x features have been removed in tox 4.x and some install behaviors changed between the two versions.
So add tox 4.x support while keeping tox >= 3.7 compatibility.