tox: define the minimal tox version required (#5914)
tox: define the minimal tox version required (#5914)


xavfernandez/pip,pradyunsg/pip,pypa/pip,rouge8/pip,pypa/pip,pfmoore/pip,sbidoul/pip,pfmoore/pip,xavfernandez/pip,sbidoul/pip,xavfernandez/pip,rouge8/pip,pradyunsg/pip,rouge8/pip