Would it really be technical debt when we talk about transition from py2->py3? Technical debt is about the consequences of bad design. But py3 is more the consequence of growth and gain of experience. And for a company it is an external debt.

I would call it more something like upgrade debt or time debt, as it's a problem that appeared because an external situation changed over time. Similar problem in that regard are operating systems which change and let old software behind, or in case of linux old packages not working anymore, repos disappearing, etc.