Support pathlib paths in NWBHDF5IO

[dsleiter]

Motivation

Fix #1303: support pathlib.Path paths when opening NWBHDF5IO objects.

Pathlib paths will be supported in the next release of hdmf (hdmf-dev/hdmf#450). This WIP PR updates pynwb to support this enhancement, and can be merged in after the next hdmf release and pip requirements are updated.

How to test the behavior?

from pathlib import Path
from pynwb import NWBHDF5IO

filepath = Path('foo.nwb')
with NWBHDF5IO(filepath, mode='w') as io:
    nwbfile = io.read()

Checklist

• Have you checked our Contributing document?
• Have you ensured the PR description clearly describes problem and the solution?
• Is your contribution compliant with our coding style? This can be checked running flake8 from the source directory.
• Have you checked to ensure that there aren't other open Pull Requests for the same change?
• Have you included the relevant issue number using #XXX notation where XXX is the issue number?

[rly]

Looks good. Thanks @dsleiter . The miniconda3.6 test is failing due to a separate issue which will be resolved in a separate PR.

[dsleiter]

Ok, thanks for taking a look at the failing test issue @rly and for the review!