spec: add pytest-cov BR
    
    The build process was failing because `pytest --cov*` arguments are unknown when
    the `pytest-cov` plugin is not installed. This will make the build process
    understand the `--cov*` args. It will not measure test coverage, so this has no
    performance impact.
    
    Fixes: https://pagure.io/fedora-qa/blockerbugs/issue/217