[tahoe-lafs-trac-stream] [Tahoe-LAFS] #3325: Add py38 to CI

[Tahoe-LAFS]

#3325: Add py38 to CI
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 Reporter:  chadwhitacre        |          Owner:
     Type:  enhancement         |         Status:  new
 Priority:  normal              |      Milestone:  Support Python 3
Component:  dev-infrastructure  |        Version:  n/a
 Keywords:                      |  Launchpad Bug:
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 Acceptance

 1. py38 is included in the test matrix for Travis (other CI services are
 out of scope for now).
 1. The test suite under py38 runs to completion (no install/setup errors).
 1. Expected successes under py38 are tracked and any variance is cause for
 failure.

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Ticket URL: <https://tahoe-lafs.org/trac/tahoe-lafs/ticket/3325>
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