pathlib ABCs: restore `relative_to()` and `is_relative_to()` (#138853)

Restore `JoinablePath.[is_]relative_to()`, which were deleted in
ef63cca494571f50906baae1d176469a3dcf8838. These methods are too useful to
forgo. Restore old tests, and add new tests covering path classes with
non-overridden `__eq__()` and `__hash__()`.

Slightly simplify `PurePath.relative_to()` while we're in the area.

No change to public APIs, because the pathlib ABCs are still private.