[3.10] Fix `PurePath.relative_to` links in the pathlib documentation (GH-93268) (GH-93590)

These are currently broken as they refer to :meth:`Path.relative_to` rather than :meth:`PurePath.relative_to`, and `relative_to` is a method on `PurePath`..
(cherry picked from commit 8ef7929baffd0038bcb38978a6a91ecc63bf0de2)

Co-authored-by: jacksonriley <52106215+jacksonriley@users.noreply.github.com>

Backport of #93268