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

See the commit messages for more detail.

Note: EDNS Client Subnet extension is enabled.