Adds support for PEP 519 File Path protocol

[jaden-young]

Allows AudioSegment.from_file() and AudioSegment.export() to accept PathLike objects that represent file system paths, in addition to strings.

Python 3.4 introduced pathlib to make handling filesystem paths easier. In Python 3.6, PEP 519 added support for a new protocol to many builtin methods, such as open, to allow objects to signal that they represent filesystem paths by returning a str or bytes from a __fspath()__ method.

>>> from pydub import AudioSegment
>>> from pathlib import Path
>>> music_folder = Path('music')
>>> song_path = music_folder / 'my_song.mp3'
>>> song_audio = AudioSegment.from_file(song_path)
>>> song_audio.export(music_folder / 'my_song_backup.mp3')

Explicit support for the protocol is necessary since the utility method _fd_or_path_or_tempfile is expected to return anything that isn't a string that represents a path unaltered, and thus explicitly checks for string.