Improve the "not in" assertion output
Improve the "not in" assertion output

This cleans up the generic diff and tailors the output more to this
specific assertion (based on feedback from hpk).

ojake/pytest,codewarrior0/pytest,inirudebwoy/pytest,txomon/pytest,icemac/pytest,Haibo-Wang-ORG/pytest,skylarjhdownes/pytest,untitaker/pytest,hackebrot/pytest,bukzor/pytest,JonathonSonesen/pytest,MengJueM/pytest,pelme/pytest,hunse/pytest,Carreau/pytest,hpk42/pytest,RonnyPfannschmidt/pytest,lukas-bednar/pytest,Bjwebb/pytest,lukas-bednar/pytest,jb098/pytest,hunse/pytest,pfctdayelise/pytest,JonathonSonesen/pytest,ionelmc/pytest,nicoddemus/pytest,tareqalayan/pytest,rmfitzpatrick/pytest,doordash/pytest,ghostsquad/pytest,markshao/pytest,pelme/pytest,userzimmermann/pytest,ddboline/pytest,etataurov/pytest,mbirtwell/pytest,mdboom/pytest,MengJueM/pytest,pytest-dev/pytest,userzimmermann/pytest,tgoodlet/pytest,mdboom/pytest,omarkohl/pytest,abusalimov/pytest,vodik/pytest,nicoddemus/repo-test,ericdill/pytest,Carreau/pytest,Bachmann1234/pytest,doordash/pytest,jb098/pytest,gabrielcnr/pytest,nicoddemus/pytest,ojake/pytest,oleg-alexandrov/pytest,omarkohl/pytest,bukzor/pytest,nicoddemus/repo-test,ionelmc/pytest,ghostsquad/pytest,ericdill/pytest,wfxiang08/pytest,ropez/pytest,takluyver/pytest,davidszotten/pytest,alfredodeza/pytest,Haibo-Wang-ORG/pytest,jaraco/pytest,hpk42/pytest,malinoff/pytest,takluyver/pytest,MichaelAquilina/pytest,vodik/pytest,tomviner/pytest,The-Compiler/pytest,wfxiang08/pytest,abusalimov/pytest,Bjwebb/pytest,chiller/pytest,chiller/pytest,chillbear/pytest,mhils/pytest,Akasurde/pytest,untitaker/pytest,rouge8/pytest,Bachmann1234/pytest,tomviner/pytest,rouge8/pytest,bubenkoff/pytest,gabrielcnr/pytest,oleg-alexandrov/pytest,flub/pytest,eli-b/pytest,chillbear/pytest,ropez/pytest,vmalloc/dessert,mhils/pytest,bubenkoff/pytest,mbirtwell/pytest,The-Compiler/pytest,inirudebwoy/pytest,icemac/pytest,codewarrior0/pytest