tox: Don't expand symlinks
    
    `virtualenv` < 20.0.0 copies system python binary into virt
    environment and then links `python` to it. While
    `virtualenv` >= 20.0.0 directly links `python` to system python
    binary (without copying).
    
    `realpath` by default expands symlinks. Thereby, pip attempts to
    install packages into the system's site-packages and
    fails with 'Permission denied' (non-privileged user).
    
    Fixes: https://pagure.io/freeipa/issue/8475
    Signed-off-by: Stanislav Levin <slev@altlinux.org>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>