[tahoe-lafs-trac-stream] [Tahoe-LAFS] #4021: Add a CI job that is explicitly for "run with latest version of dependencies"

[Tahoe-LAFS]

#4021: Add a CI job that is explicitly for "run with latest version of
dependencies"
---------------------+---------------------------
 Reporter:  exarkun  |          Owner:
     Type:  defect   |         Status:  new
 Priority:  normal   |      Milestone:  undecided
Component:  unknown  |        Version:  n/a
 Keywords:           |  Launchpad Bug:
---------------------+---------------------------
 * It should run on a schedule, not as part of normal PR workflow.
 * It should be clearly marked as the "latest version" job so failures from
 it are more easily interpreted.
 * Its results should go *somewhere* so they're not overlooked (as surely
 will be the default outcome if they're not part of the normal PR
 workflow).
 * It should obey constraints in our metadata (so if we rule _out_ a new
 version intentionally, we don't get it).
 * It should have a lot of retries so it doesn't fail because of a network
 error downloading new packages or a pypi API hiccup.

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Ticket URL: <https://tahoe-lafs.org/trac/tahoe-lafs/ticket/4021>
Tahoe-LAFS <https://Tahoe-LAFS.org>
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