hotfix: drop bad pyyaml version

[Borda]

What does this PR do?

just a hotfix for the recent pyyaml release which seems to work badly with Namespace
see: yaml/pyyaml#482

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[codecov]

Codecov Report

Merging #5606 (8a1b059) into master (e1c152b) will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #5606   +/-   ##
======================================
  Coverage      93%     93%           
======================================
  Files         135     135           
  Lines       10005   10005           
======================================
  Hits         9339    9339           
  Misses        666     666