This module offers classes representing filesystem paths with semantics appropriate for different operating systems. Path classes are divided between pure paths, which provide purely computational operations without I/O, and concrete paths, which inherit from pure paths but also provide I/O operations.
という事で、今回のPurePathはPathをつくるだけの関数のようです。
PurePath("/hoge","bar/buzz") # PurePosixPath('/hoge/bar/buzz')
賢いので、"."であれば無視してくれます
PurePath("hoge",".","bar/buzz") # PurePosixPath('hoge/bar/buzz')
11.1. pathlib — Object-oriented filesystem paths — Python 3.6.4 documentation