[tahoe-lafs-trac-stream] [Tahoe-LAFS] #3889: update nix packaging to latest nixos 21.11 / pypi db

[Tahoe-LAFS]

#3889: update nix packaging to latest nixos 21.11 / pypi db
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 Reporter:  exarkun  |          Owner:
     Type:  defect   |         Status:  new
 Priority:  normal   |      Milestone:  undecided
Component:  unknown  |        Version:  n/a
 Keywords:           |  Launchpad Bug:
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 nixos 21.11 now has zfec with some fixes for compiler warnings and for the
 argparse packaging error

 pypi now has pycddl

 it would be nice to make both of these things available to our nix package

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Ticket URL: <https://tahoe-lafs.org/trac/tahoe-lafs/ticket/3889>
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