allow pdbcls without implying usepdb
allow pdbcls without implying usepdb

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