tests: use fixed version for idna in plainbox

[sergiusens]

Signed-off-by: Sergio Schvezov sergio.schvezov@canonical.com

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[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@99b7cc9). Click here to learn what that means.
The diff coverage is n/a.

@@           Coverage Diff            @@
##             master   #2426   +/-   ##
========================================
  Coverage          ?   90.1%           
========================================
  Files             ?     196           
  Lines             ?   12891           
  Branches          ?    1956           
========================================
  Hits              ?   11615           
  Misses            ?     876           
  Partials          ?     400

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