Stop using sphinxcontrib-issuetracker for docs.
    
    The sphinxcontrib-issuetracker is no longer maintained and is not
    compatible with latest Sphinx release. See this bug for more details:
    
    https://github.com/ignatenkobrain/sphinxcontrib-issuetracker/issues/23
    
    This Sphinx extension is supposed to replace #bugid with link pointing
    to Pagure issue page. We have never been using that as far as I know.