def relto(self, relpath):
        """Return a string which is the relative part of the path
        to the given 'relpath'.
        """
        if not isinstance(relpath, (str, LocalPath)):
            raise TypeError(f"{relpath!r}: not a string or path object")
        strrelpath = str(relpath)
        if strrelpath and strrelpath[-1] != self.sep:
            strrelpath += self.sep
        # assert strrelpath[-1] == self.sep
        # assert strrelpath[-2] != self.sep
        strself = self.strpath
        if sys.platform == "win32" or getattr(os, "_name", None) == "nt":
            if os.path.normcase(strself).startswith(os.path.normcase(strrelpath)):
                return strself[len(strrelpath) :]
        elif strself.startswith(strrelpath):
            return strself[len(strrelpath) :]
        return ""
Return a string which is the relative part of the path
        to the given 'relpath'.
        
src/_pytest/_py/path.py