tox.ini: add config for isort
tox.ini: add config for isort

nicoddemus/pytest,nicoddemus/pytest,markshao/pytest,tomviner/pytest,Akasurde/pytest,pytest-dev/pytest,tomviner/pytest,RonnyPfannschmidt/pytest,The-Compiler/pytest,The-Compiler/pytest,alfredodeza/pytest