testing: add a file for checking no mypy errors
testing: add a file for checking no mypy errors

We probably something a bit more elaborate in the future but for now
it's something to verify fixes and catch regressions.

RonnyPfannschmidt/pytest,pytest-dev/pytest,nicoddemus/pytest,Akasurde/pytest,nicoddemus/pytest,The-Compiler/pytest,markshao/pytest,The-Compiler/pytest