tox: remove obsolete whitelist_externals
tox: remove obsolete whitelist_externals

nicoddemus/pytest,alfredodeza/pytest,The-Compiler/pytest,markshao/pytest,tomviner/pytest,RonnyPfannschmidt/pytest,Akasurde/pytest,davidszotten/pytest,hackebrot/pytest,The-Compiler/pytest,tomviner/pytest,pfctdayelise/pytest,nicoddemus/pytest,ddboline/pytest,pytest-dev/pytest,txomon/pytest