[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
 Keywords:                      |  Launchpad Bug:
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 In #3405 we want to add Python 3.6 on Windows to CI. We have two CI
 services, CircleCI and !GitHub Actions. We use the former for exercising
 the test suite, so it seems that we should try to continue using it for
 the same on Windows. The easiest (only?) way to use Windows on CircleCI is
 via features—[https://circleci.com/docs/2.0/configuration-reference/#orbs-
 requires-version-21 orbs] and [https://circleci.com/docs/2.0
 /configuration-reference/#executors-requires-version-21 executors]—that
 are only available with version 2.1 of their configuration format.
 However, there are some ''other'' changes with v2.1 that we'll have to
 take on board first, not least of which is an approach to
 [https://circleci.com/docs/2.0/reusing-config/#notes-on-reusable-
 configuration config reusability] that imposes new restrictions on job
 names: [https://discuss.circleci.com/t/v2-1-job-name-validation/31123 no
 more dots]!

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Ticket URL: <https://tahoe-lafs.org/trac/tahoe-lafs/ticket/3406>
Tahoe-LAFS <https://Tahoe-LAFS.org>
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