Third Party: Switch to eopkg.py2 for building packages

[Staudey]

Description

This makes the instructions compatible with the recent switch to Python 3 (Nuitka) eopkg by default

Prevents the following error:

sudo eopkg bi --ignore-safety pspec.xml
Program terminated.
Building packages is not supported when eopkg is compiled with nuitka. Please use eopkg.py3 instead.

[Staudey]

I've discovered that eopkg.py3 is not part of the default installation as it is in the python-eopkg subpackage, not in the main eopkg package. I think asking people to install that might be the only way to make this work, unless someone finds out how to properly fix getsolus/eopkg#49 instead of disabling it in the nuitka build (I took a look for half an hour or so yesterday but got nowhere)

[Staudey]

Actually we can just make it use eopkg.py2 for this for now

[Staudey]

Okay, this is good to go and can actually be merged at any time before the sync too.