ipalib/x509.py: get rid of unicode helper
Pylint started to complain that 'unicode' variable is accessed before
definition. This is clearly a bug in how 'six' and pylint are working
together.
Replace use of 'unicode()' by 'str()'.
Fixes: https://pagure.io/freeipa/issue/9644
Signed-off-by: Alexander Bokovoy <abokovoy@redhat.com>
Reviewed-By: Rob Crittenden <rcritten@redhat.com>