[9.18] chg: test: Support dnspython 2.7.0

Closes #4988

Backport of MR !9634

Merge branch 'backport-4988-dnspython-2.7.0-support-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!9636

bin/tests/system/cookie/ans9/ans.py

@@ -121,10 +121,14 @@ def create_response(msg, tcp, first, ns10):
                     )
                 elif labels[0] != "tcponly" or tcp:
                     cookie = o
-                    if len(o.data) == 8:
-                        cookie.data = o.data + o.data
-                    else:
-                        cookie.data = o.data
+                    try:
+                        if len(o.server) == 0:
+                            cookie.server = o.client
+                    except AttributeError:  # dnspython<2.7.0 compat
+                        if len(o.data) == 8:
+                            cookie.data = o.data + o.data
+                        else:
+                            cookie.data = o.data
                     r.use_edns(options=[cookie])
     r.flags |= dns.flags.AA
     return r