Py3: Replace six.string_types with str

In Python 3, six.string_types is just an alias for str.

See: https://pagure.io/freeipa/issue/7715
Signed-off-by: Christian Heimes <cheimes@redhat.com>
Reviewed-By: Florence Blanc-Renaud <frenaud@redhat.com>

ipalib/aci.py

@@ -165,7 +165,7 @@ class ACI:
                 raise SyntaxError("invalid permission: '%s'" % p)
         if not self.name:
             raise SyntaxError("name must be set")
-        if not isinstance(self.name, six.string_types):
+        if not isinstance(self.name, str):
             raise SyntaxError("name must be a string")
         if not isinstance(self.target, dict) or len(self.target) == 0:
             raise SyntaxError("target must be a non-empty dictionary")