[tahoe-lafs-trac-stream] [Tahoe-LAFS] #3619: Enable BytesWarnings-is-exception for test_system.py and integration tests

[Tahoe-LAFS]

#3619: Enable BytesWarnings-is-exception for test_system.py and integration tests
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 Reporter:  itamarst  |          Owner:  itamarst
     Type:  defect    |         Status:  new
 Priority:  normal    |      Milestone:  Support Python 3
Component:  unknown   |        Version:  n/a
 Keywords:            |  Launchpad Bug:
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 As follow-up to https://tahoe-lafs.org/trac/tahoe-lafs/ticket/3618, we
 should also convert `str(b"")`, `"%s" % (b"a")` etc. into exceptions when
 running end-to-end tests with subprocesses.

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Ticket URL: <https://tahoe-lafs.org/trac/tahoe-lafs/ticket/3619>
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