powerdns: target Python 3 and upgrade to 4.2.1
This MR includes the following steps:
- Target Python 3
- Upgrade powerdns from version 3.3.1 to version 4.2.1 --- which also means...
- Rework the configuration to support the new GeoIP backend config format
See the commit messages for more detail.
Note: EDNS Client Subnet extension is enabled.