Port to non-deprecated PyYAML functions.

This should also correct the breakage which is currently causing all Frameworks Linux builds to have an UNSTABLE (failing tests) status as the YAML depreciation warnings were causing the check-platforms logic to trip.

CCMAIL: kde-frameworks-devel@kde.org
2 jobs for master
Status Job ID Name Coverage
  Build
manual #3538
docker allowed to fail manual
kdeorg/appimage-base
manual #3539
docker allowed to fail manual
kdeorg/staticweb