Skipping the hostkey verification in dput/sftp

I would like to use [dput](https://packages.debian.org/jessie-backports/dput-ng) to upload a package to a launchpad PPA via sftp. After preparing the data, it all works fine until please login: To accept ssh-rsa hostkey 6b03de9833252318a646b34722cd54f2 for ppa.launchpad.net type "yes": [yes, no]: Typing yes does the trick, but is impossible to do as a cronjob. How to skip the hostkey verification in dput?