Leapp のアップグレードがエラー "DNF execution failed with non zero exit code." で失敗する

Issue

• leapp upgrade コマンドを実行すると、コマンドは失敗し、レポートには conflicts に関連する問題が表示されます。たとえば、/var/log/leapp/report.txt には以下が表示されます。

  Risk Factor: high (error)
  Title: DNF execution failed with non zero exit code.
  Summary: {"STDERR": "Warning: Package marked by Leapp to upgrade not found in repositories metadata: gpg-pubkey leapp-upgrade-el7toel8 leapp python2-leapp\nRPM: warning: Generating 6 missing index(es), please wait...\nError: Transaction test error:\n  file /usr/lib/python3.6/site-packages/__pycache__/six.cpython-36.opt-1.pyc from install of python3-six-1.11.0-8.el8.noarch conflicts with file from package python36-six-1.14.0-3.el7.noarch\n  file /usr/lib/python3.6/site-packages/__pycache__/six.cpython-36.pyc from install of python3-six-1.11.0-8.el8.noarch conflicts with file from package python36-six-1.14.0-3.el7.noarch\n  file /usr/lib/python3.6/site-packages/six.py [...]