Path objects do not support membership testing

[kouralex]

I recently found out that whilst URIRefs support expressions such as uriref in list/dict the same does not apply to Path objects, effectively making one unable to use Paths as dict keys, for instance. One could, of course, apply a .n3() string transformation whilst setting the key value in order to achieve such feature, but to me it would seem a more appropriate to be able to check for membership directly via Path objects.

[aucampia]

As @alhyss said, this seems related to #2281. I will look at it when I have time and hopefully release it in 6.3.2.