JSON encoder: add support to os.PathLike objects

[Rubo3]

Since the PathLike class is implemented in the standard os module, it would be useful to add support to this type of objects when encoding in JSON format.

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[isidentical]

This requires a bugs.python.org entry and discussion. Also str(o) is not the right way, please check out os.fspath (also handle the cases where it might return bytes)