[tahoe-lafs-trac-stream] [Tahoe-LAFS] #3431: Port test_immutable

[Tahoe-LAFS]

#3431: Port test_immutable
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 Reporter:  chadwhitacre  |          Owner:
     Type:  task          |         Status:  new
 Priority:  normal        |      Milestone:  Support Python 3
Component:  code          |        Version:  n/a
 Keywords:                |  Launchpad Bug:
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 == Acceptance

 1. `trial allmydata.test.test_immutable` passes under all testenvs

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Ticket URL: <https://tahoe-lafs.org/trac/tahoe-lafs/ticket/3431>
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