Add pathlib.PurePath support for Graph.serialize and Graph.parse

[aucampia]

Graph.parse did already support pathlib.Path but there is no good reason to not support pathlib.PurePath AFAICT.

Fixes #1227

Proposed Changes

• Add pathlib.PurePath support to Graph.serialize
• Change pathlib.Path check for Graph.parse to instead check for pathlib.PurePath

[coveralls]

Coverage increased (+0.08%) to 75.607% when pulling cfd8c33 on iafork:support_purepath into 8d592a2 on RDFLib:master.

[nicholascar]

Thanks for picking up one of my issues!

[nicholascar]

@aucampia can you resolve conflicts here so we can merge?

[aucampia]

@nicholascar resolved