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 e0bc8ee945af96f9395659bbe3cc30b082e7a361)

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>