[tahoe-lafs-trac-stream] [Tahoe-LAFS] #3406: Upgrade CircleCI config to version 2.1

[Tahoe-LAFS]

#3406: Upgrade CircleCI config to version 2.1
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     Reporter:  chadwhitacre        |      Owner:
         Type:  task                |     Status:  new
     Priority:  normal              |  Milestone:  Support Python 3
    Component:  dev-infrastructure  |    Version:  n/a
   Resolution:                      |   Keywords:
Launchpad Bug:                      |
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Comment (by exarkun):

 The ticket description doesn't describe the current situation quite
 correctly.  We currently use GitHub Actions to run both test suites on
 both Windows and macOS:

 * https://github.com/tahoe-lafs/tahoe-
 lafs/blob/23e1223c94330741f5b1dda476c3aeb42c3a012f/.github/workflows/ci.yml#L52-L53
 * https://github.com/tahoe-lafs/tahoe-
 lafs/blob/23e1223c94330741f5b1dda476c3aeb42c3a012f/.github/workflows/ci.yml#L120-L121

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Ticket URL: <https://tahoe-lafs.org/trac/tahoe-lafs/ticket/3406#comment:1>
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