minor: improve FixtureDef.__repr__
minor: improve FixtureDef.__repr__

Ref: https://github.com/pytest-dev/pytest/pull/4105#pullrequestreview-163486766

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