Let mkdir() and mkpydir() receive PathLike names
Let mkdir() and mkpydir() receive PathLike names

These pytester utility methods were annotated to only receive `str`
names, but they naturally support os.PathLike values, as well.

This makes writing some pytester calls a little nicer, such as when
creating a directory based on a `.joinpath()` call. We previously needed
to cast that intermediate value to a `str`.

markshao/pytest,pytest-dev/pytest,RonnyPfannschmidt/pytest