ipasetup: fix dependencies handling based on python version
    
    On RHEL system ipasetup is failing:
    ```
    ValueError: (":python_version>='3'", ['pyldap'])
    ```
    
    Our RHEL re-implementation of setuptools logic should ignore python3
    packages when python3 is not available.
    
    https://pagure.io/freeipa/issue/6875
    
    Reviewed-By: Pavel Vomacka <pvomacka@redhat.com>