[9.18] Bump the requirement in the shutdown test to dnspython 2.0.0
The dnspython.Resolve.resolve() requires at least dnspython >= 2.0.0, this wasn't enforced in the shutdown system test leading to infinite loop waiting for the server start due to failing resolve() call.
Backport of MR !7796 (merged)
Edited by Ondřej Surý