In my testing the puppetlabs PuppetDB 4.4 package for jessie works as-is on stretch as well. Since (according to puppetlabs docs) 4.4 is the latest version of PuppetDB still supporting puppet 4, and there is not yet a stable Debian PuppetDB 4.4 package, let's port the puppetlabs jessie package to stretch.
Let's also think about conflicts that may arise in the future if/when a proper Debian PuppetDB package becomes available.