CI: Remove `pipenv --three` from tests
    
    It does no longer exist:
    
        Usage: pipenv [OPTIONS] COMMAND [ARGS]...
        Try 'pipenv -h' for help.
    
        Error: No such option: --three (Possible options: --bare, --help, --where)
    
    The `pipenv install` command will create the environment.
    See https://src.fedoraproject.org/rpms/pipenv/c/5591b2df503495b