[tahoe-lafs-trac-stream] [Tahoe-LAFS] #3653: Use newer coveralls version in GitHub Actions

[Tahoe-LAFS]

#3653: Use newer coveralls version in GitHub Actions
---------------------+---------------------------
 Reporter:  sajith   |          Owner:
     Type:  defect   |         Status:  new
 Priority:  normal   |      Milestone:  undecided
Component:  unknown  |        Version:  n/a
 Keywords:           |  Launchpad Bug:
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 This is related to [https://github.com/tahoe-lafs/tahoe-lafs/pull/986 the
 discussion] about #3616.

 While working on #3616, it turned out that coveralls-python (henceforth
 "coveralls") maintains two separate, slightly incompatible, versions for
 Python 2 and Python 3.  At this time of writing,  coveralls 1.11.1 will be
 installed for Python 2.7, and coveralls 3.0.1 will be installed for Python
 3.x.

 The newer version of coveralls has a `--finish` option, which is useful to
 indicate to coveralls.io that a number of parallel builds have finished.
 Without this option, we will have to manually construct some parameters
 and use `curl` to do the same, as done here:

 https://github.com/LeastAuthority/tahoe-
 lafs/blob/786433056231320ef994b63ff7b61bc3ff9e940c/.github/workflows/ci.yml#L107-L185

 That should be fine, except when CI needs to run Python 2.7 tests and
 Python 3.x tests in parallel.  In the same CI run, IDs generated by the
 two versions of coveralls will be of different types: older coveralls gets
 something like `cc6f006336de603b95c183e4e80e059b88f20b8d-PR-986.2`, and
 newer one gets `630187837.1`. If we mix coveralls 1.x and 3.x in the same
 CI run, coveralls.io will have trouble matching up the jobs, and will
 never send coverage status back to !GitHub.

 We could invoke the newer version of coveralls uniformly, relying on the
 fact that !GitHub Actions tool cache already has Python3 and pip3, even
 when a CI job is set up to run Python 2.7 tests.

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Ticket URL: <https://tahoe-lafs.org/trac/tahoe-lafs/ticket/3653>
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