sudocmd: fix unsupported assignment

sudocmd.get_dn() was trying to assign in an item of a tuple
which is not possible.

https://pagure.io/freeipa/issue/6874

Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>

ipaserver/plugins/sudocmd.py

@@ -126,7 +126,7 @@ class sudocmd(LDAPObject):
 
     def get_dn(self, *keys, **options):
         if keys[-1].endswith('.'):
-            keys[-1] = keys[-1][:-1]
+            keys = (keys[:-1] + (keys[-1][:-1], ))
         dn = super(sudocmd, self).get_dn(*keys, **options)
         try:
             self.backend.get_entry(dn, [''])