Fix create_unix_server to support Path-like objects (PEP 519)

[1st1]

One of the recent commits broke support of PEP 519. This PR fixes loop.create_unix_server to support Path-like objects + a regression test.

cc @brettcannon

[asvetlov]

Maybe support pathlib.Path for Python 3.5 also?
It could be implemented by isinstance() check.

[1st1]

I don't think it's a good idea, PEP 519 is 3.6 only.

[brettcannon]

I think the key point is that unless a library implementing a path-representing object restricts itself to 3.6 or newer, it could be useful in 3.5 code. But I wouldn't use isinstance() as that's too restrictive if you want to bother supporting __fspath__. All you really need to be good enough for a os.fspath() fall-back is:

def _fspath(path):
  if hasattr(path, '__fspath__'):
    return path.__class__.__fspath__(path)
  else:
    return path

os.fspath() does more checks to verify that only str and bytes are returned, but that's the gist of it. Otherwise just do what Yury did.

[asvetlov]

ok

[1st1]

Pushed in fff05d4