[tahoe-lafs-trac-stream] [Tahoe-LAFS] #4101: Work with latest attrs

[Tahoe-LAFS]

#4101: Work with latest attrs
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 Reporter:  itamarst  |          Owner:
     Type:  defect    |         Status:  new
 Priority:  normal    |      Milestone:  undecided
Component:  unknown   |        Version:  n/a
 Keywords:            |  Launchpad Bug:
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 attrs removed attr.validators.provides(), so we need to use our own copy
 of that (allmydata.utils.attrs_provides.provides).

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Ticket URL: <https://tahoe-lafs.org/trac/tahoe-lafs/ticket/4101>
Tahoe-LAFS <https://Tahoe-LAFS.org>
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