[Backport maintenance/4.0.x] Skip mypy install for pypy (#2906) Skip mypy install for pypy and mypy tests if it's not installed (#2905) (cherry picked from commit 723c66b9f1cf9b503c88ae794f8a752cfc90db57) Co-authored-by: Marc Mueller <30130371+cdce8p@users.noreply.github.com>