epoch prerequisite: Switch BuildXML function to use eopkg.py3

[ermo]

3rd party packages builds use BuildXML.

After the epoch jump, eopkg.py2 (and Solus SC) will no longer be present, but it will still be possible to build the ported-to-py3 3rd party package recipes w/eopkg.py3, assuming that each recipe is successfully ported to use a #!/usr/bin/env/python3 shebang.

@Staudey has a PR for this here