Accept os.PathLike paths in every S3 filesystem method #1380

Part of #1341 (decision 9 of the audit).

Current state (29cc1d3)

With pathlib.Path("bucket/a"):

1. chmod, cat_file, rm_file, touch, metadata, sign, getxattr, setxattr, get_tags, object_version_info, cp_file, list_multipart_uploads and put_file (remote path) raise TypeError: expected string or bytes-like object from S3Path.parse.
2. rm raises TypeError: 'PosixPath' object is not iterable.
3. glob raises AttributeError (endswith).
4. info, exists, ls, find, du, size and checksum work, through fsspec's _strip_protocol, which calls stringify_path.

Decision

Every public method of S3FileSystem and AioS3FileSystem converts an os.PathLike path with fsspec.utils.stringify_path before it classifies the path: rm's iterable check, glob, and the pairing's isinstance(..., str) decisions included. Slashes are not stripped by the conversion.