bpo-46483: [doc] pathlib classes no longer support parameterized generics (GH-31281)

Remove pathlib classes from the list in stdtypes.rst of classes
that can be parameterized at runtime.
(cherry picked from commit e0bc8ee)

Co-authored-by: Alex Waygood <[email protected]>

Author: miss-islington

Doc/library/stdtypes.rst

@@ -4903,10 +4903,6 @@ list is non-exhaustive.
 * :class:`functools.cached_property`
 * :class:`functools.partialmethod`
 * :class:`os.PathLike`
-* :class:`pathlib.Path`
-* :class:`pathlib.PurePath`
-* :class:`pathlib.PurePosixPath`
-* :class:`pathlib.PureWindowsPath`
 * :class:`queue.LifoQueue`
 * :class:`queue.Queue`
 * :class:`queue.PriorityQueue`