[9.20] fix: test: Require dnspython>2.0.0 in system tests using asyncserver

[9.20] fix: test: Require dnspython>2.0.0 in system tests using asyncserver

Maintaining compatibility with pre-2.0.0 dnspython became cumbersome
leading to failure in nightly CI jobs which are the only ones that run
with dnspython this old.

Abort all AsyncServer instances when running with old dnspython. Add an
importor skip for all system tests using isctest.asyncserver.

Full removal of pre-2.0.0 `dnspython` support is planned for after 9.18
goes EoL.

Backport of MR !11191

Merge branch 'backport-stepan/require-dnspython-2-for-asyncserver-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!11209