Use fspath to consistently support PathLike objects as input/output filenames.

Path-like objects were already supported by input and by output when the filename was explicitly specified as a keyword argument. (subprocess.Popen accepts path-like objects in addition to strings.) However, implicitly specifying the output filename as the last element of a list of output streams/filename did not permit a path-like object; instead, this raises a ValueError with a somewhat misleading error message.

>>> import ffmpeg
>>> from pathlib import Path
>>> ffmpeg.input("input.mp4").output("output.mp4")
output(filename='output.mp4')[None] <1fa880dddf3d>
>>> ffmpeg.input("input.mp4").output(filename="output.mp4")
output(filename='output.mp4')[None] <1fa880dddf3d>
>>> ffmpeg.input(Path("input.mp4")).output(filename="output.mp4")
output(filename='output.mp4')[None] <3105870c5a1e>
>>> ffmpeg.input(Path("input.mp4")).output(filename=Path("output.mp4"))
output(filename=PosixPath('output.mp4'))[None] <3cf3744481bb>
>>> ffmpeg.input(Path("input.mp4")).output(Path("output.mp4"))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File ".../ffmpeg/_ffmpeg.py", line 85, in output
    raise ValueError('A filename must be provided')
ValueError: A filename must be provided

This PR uses a patched version of os.fspath to explicitly convert all path-like objects to strings (or raise an error). Once support is dropped for versions before 3.6, this should be updated to use os.fspath directly.