Accept pathlib.Path objects for stdout/stderr

[MarSoft]

Fix #597.
I decided to not add an additional import for pathlib.Path and instead check if the object passed is "path-like" according to PEP-519.

[amoffat]

Thanks for documenting the issue and starting a PR. Would you mind adding some tests around the usage of a Path object please?

[mentalisttraceur]

Code review nit: the Pythonic way to resolve magic methods is to look for them on the class, not on the instance:

-hasattr(out, '__fspath__')
+hasattr(type(out), '__fspath__')

Well, even more Pythonic is to just use isinstance with the abstract base class:

isinstance(out, os.PathLike)

[mentalisttraceur]

Based on the comment in #579 about still supporting Python 2, isinstance(out, os.PathLike) is out, so hasattr(type(out), '__fspath__') is probably the closest to the Right(tm) test we can do with the same code on both 2 and 3.

[ecederstrand]

I've added a test case now.

While Python 3.5 introduced pathlib, __fspath__ wasn't introduced until Python 3.6. Therefore, using Path objects on Python 3.5 still won't work. I think that's an acceptable discrepancy.