argument parsing: always warn for string types
argument parsing: always warn for string types

fix #1741

The-Compiler/pytest,malinoff/pytest,flub/pytest,tomviner/pytest,hackebrot/pytest,tgoodlet/pytest,alfredodeza/pytest,txomon/pytest,jaraco/pytest,etataurov/pytest,ddboline/pytest,davidszotten/pytest,nicoddemus/pytest,pytest-dev/pytest,tomviner/pytest,skylarjhdownes/pytest,nicoddemus/pytest,pfctdayelise/pytest,eli-b/pytest,hpk42/pytest,vmalloc/dessert,RonnyPfannschmidt/pytest,rmfitzpatrick/pytest,The-Compiler/pytest,Akasurde/pytest,markshao/pytest,hpk42/pytest,MichaelAquilina/pytest,tareqalayan/pytest