chore: bump dnspython to fix vulnerability (#2255)

Author: dpgaspar

requirements/base.txt

@@ -22,7 +22,7 @@ colorama==0.4.6
     # via flask-appbuilder
 deprecated==1.2.14
     # via limits
-dnspython==2.4.2
+dnspython==2.6.1
     # via email-validator
 email-validator==1.3.1
     # via flask-appbuilder