Support recent versions of pytest

[joaander]

Description

• Simplify test code using pytest fixtures and parameterize features.
• Support pathlib paths

Motivation and Context

Pytest removed yield based tests, this prevented new versions of pytest from running all of gsd's tests. Going forward, pytest's fixture and parameterize features greatly simplify the test code, making it easier to read and write - at the expense of requiring pytest for testing and no longer supporting nose.

Specifically, pytest >= 3.9 is needed for tmp_path support.

How Has This Been Tested?

CI tests pass with pytest 5.

Change log

* pytest >= 3.9.0 is required to run unit tests
* ``gsd.fl.open`` and ``gsd.hoomd.open`` accept objects implementing ``os.PathLike``

Checklist:

• I have reviewed the Contributor Guidelines.
• I agree with the terms of the GSD Contributor Agreement.
• My name is on the list of contributors.
• I am a member of the gsd-contributors team.

[bdice]

Looks good to me.

[lyrivera]

I'm not too familiar with CI configurations yet, but the actual tests look good.