Modify dns_resolver_test to skip tests if dnspython is not available.
Modify dns_resolver_test to skip tests if dnspython is not available.

jtl999/certbot,lmcro/letsencrypt,stweil/letsencrypt,letsencrypt/letsencrypt,letsencrypt/letsencrypt,jsha/letsencrypt,jsha/letsencrypt,bsmr-misc-forks/letsencrypt,lmcro/letsencrypt,bsmr-misc-forks/letsencrypt,stweil/letsencrypt,jtl999/certbot