IrodsPath.parent should be calculated from absolute path

[chStaiger]

My irods_home is "/nluu12p/home/research-test-christine" and I can instantiate it in three ways.

In [5]: print(IrodsPath(session).parent)
/nluu12p/home/research-test-christine

In [6]: print(IrodsPath(session, "~").parent)
/nluu12p/home/research-test-christine

In [7]: print(IrodsPath(session, session.home).parent)
/nluu12p/home

But only the last one will give me the correct parent.

[qubixes]

It's the old pathlib doing it's things again. I guess we're going to move further and further away from copying its behavior..

Will fix after #169 is merged.