Allow parsing Nodes as PathLike

[Repiteo]

Adds __fspath__ to the Node class so they can be passed directly as pathlike arguments.

Contributor Checklist:

• I have created a new test or updated the unit tests to cover the new/changed functionality.
• I have updated CHANGES.txt and RELEASE.txt (and read the README.rst).
• I have updated the appropriate documentation

[mwichmann]

This looks reasonable to me, after scanning through it in more detail than the first glance.

I'll want to think about where this has doc impacts, I think we talk about using str() to get the file name string somewhere. Being a PathLike object means that's handled for file-ish uses such open, checks like isdir, etc. but doesn't work when you need to get the string to do something else with (like printing for debug - that's the most frequent use by far in the docs). I see that came up a few times in the tests. Probably the Node Objects section.

[mwichmann]

Adding for reviewer reference - the protocol used came in via PEP 519, introduced in Python 3.6.