powerdns: target Python 3 and upgrade to 4.2.1

This MR includes the following steps:
1. Target Python 3
2. Upgrade powerdns from version 3.3.1 to version 4.2.1 --- which also means...
3. Rework the configuration to support the [new GeoIP backend config format](https://doc.powerdns.com/authoritative/backends/geoip.html)

See the commit messages for more detail.

Note: EDNS Client Subnet extension is enabled.

See merge request !764