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.

bin/tests/system/shutdown/tests_shutdown.py

@@ -21,7 +21,7 @@ import time
 
 import pytest
 
-pytest.importorskip("dns")
+pytest.importorskip("dns", minversion="2.0.0")
 import dns.exception
 import dns.resolver