Don't treat warnings as errors
    
    To workaround:
    
    Warning, treated as error:
    /builddir/build/BUILD/Python-3.9.0rc2/Doc/library/string.rst:311:duplicate token description of sign, other instance in library/functions
    
    Proper fix is in progress in upstream:
    
    https://github.com/python/cpython/pull/22281