Remove usage of `PosixPath.is_relative_to`
Remove usage of `PosixPath.is_relative_to`

Remove usage of `PosixPath.is_relative_to` because it was added in
Python 3.9 and earlier versions of Python are supported by `GitPython`.

gitpython-developers/GitPython,gitpython-developers/gitpython,gitpython-developers/gitpython,gitpython-developers/GitPython