ioPath - remove @dataclass decorator to maintain Python 3.6 compatibility
ioPath - remove @dataclass decorator to maintain Python 3.6 compatibility

Summary: Previously used `dataclass` in `NonBlockingIO`. Changed to `collections.namedtuple`.

Reviewed By: sujitoc

Differential Revision: D26925003

fbshipit-source-id: e8dfefca4363ab35901c8359a0aa6d0e0d7ef33d

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