API: Add support for PEP 519

[AnkurDedania]

File System Path Protocol has been accepted and implemented into Python 3.6, a backwards compatibility layer to support the introduced PEP 519 for python version < 3.6.

[shoyer]

See this section for guidance relevant to pandas:
https://www.python.org/dev/peps/pep-0519/#backwards-compatibility

Libraries wishing to support path objects and a version of Python prior to Python 3.6 and the existence of os.fspath() can use the idiom of path.__fspath__() if hasattr(path, "__fspath__") else path .

[flying-sheep]

so we add to compat/__init__.py (with extracted stuff from here)

def fspath(path):
	if isinstance(path, (str, bytes)):
 		return path
	
	if _PATHLIB_INTSTALLED and isinstance(path, pathlib.Path):
		path = text_type(path)  # 3.4/3.5 compat
	if _PYPATH_INSTALLED and isinstance(path, LocalPath):
		path = path.strpath
	else:
		path_type = type(path)
		try:
			path = path_type.__fspath__(path)
		except AttributeError:
			if hasattr(path_type, '__fspath__'):
				raise
			raise TypeError("expected str, bytes, py.path.LocalPath or os.PathLike object, not " + path_type.__name__) from None
	
	if isinstance(path, (str, bytes)):
		return path
	else:
		raise TypeError("expected __fspath__() to return str or bytes, not " + type(path).__name__)

[jreback]

there are already routines in io/common.py to handle pathlike things - they just need some updating