Provide cairocffi[xcb] from the python3-cairocffi package
    
    The "xcb" extra can be installed via pip, but when the package is built,
    it is either built with it, or without it.
    Since we BuildRequire python3-xcffib, the package is built with the extra.
    Hence, having a separate subpackage for cairocffi[xcb] does not make sense.
    
    However, other packages can require python3.Xdist(cairocffi[xcb]),
    currently, qtile 0.14.2 requires python3.Xdist(cairocffi[xcb]) >= 0.9.
    For that, we manually add the provides to the "main" python3-cairocffi package.
    
    Beware that the version of the provide is naïve: it is %{version}.
    The automatic python3dist() provides convert the version to a more Python
    compatible format (such as stripping the trailing .0),
    but mimicking that would be currently tedious.
    If this prove problematic in the future, a mechanism can be implemented to
    convert the version.
    
    As a reference, see the discussion in https://src.fedoraproject.org/rpms/python-cairocffi/pull-request/2