pylint: Skip unused-private-member for unsupported cases

> This mangling is done without regard to the syntactic position of the
identifier, as long as it occurs within the definition of a class.

`__set_attr` is called for instance of the class within its
classmethod.

Fixes: https://pagure.io/freeipa/issue/9117
Signed-off-by: Stanislav Levin <slev@altlinux.org>
Reviewed-By: Rob Crittenden <rcritten@redhat.com>

ipapython/cookie.py

@@ -444,7 +444,7 @@ class Cookie:
             except Exception:
                 raise ValueError("Max-Age value '%s' not convertable to integer" % value)
 
-    def __set_attr(self, name, value):
+    def __set_attr(self, name, value):  # pylint: disable=unused-private-member
         '''
         Sets one of the predefined cookie attributes.
         '''