don't fail the RPM build on Sphinx warnings
    
    Now that we are using sphinxcontrib-issuetracker, some warnings are
    unfortunately expected when we build inside Koji, because it won't be
    able to reach the network:
    
        WARNING: GET https://pagure.io/api/0/waiverdb/issue/73 failed with error: ...
    
    We can just ignore them. The Jenkins pipeline will still build the docs
    with SPHINXOPTS=-W so we will still notice any *other* warnings that
    crop up.