Prepare CHANGELOG for version 3.1.0
Prepare CHANGELOG for version 3.1.0

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