[tahoe-lafs-trac-stream] [Tahoe-LAFS] #3394: Fix test failures on tahoe --version and Python 3.7 and 3.8

[Tahoe-LAFS]

#3394: Fix test failures on tahoe --version and Python 3.7 and 3.8
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     Reporter:  jaraco       |      Owner:
         Type:  enhancement  |     Status:  new
     Priority:  normal       |  Milestone:  Support Python 3
    Component:  code         |    Version:  n/a
   Resolution:               |   Keywords:
Launchpad Bug:               |
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Comment (by jaraco):

 To be clear, I understand that Python 3.6 is the target. These changes
 will add forward compatibility easing development, but acknowledging that
 Python 3.6 is still the target.

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Ticket URL: <https://tahoe-lafs.org/trac/tahoe-lafs/ticket/3394#comment:1>
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