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.
Staging instance, all changes can be removed at any time
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.