The only software I've written in Python that's been sold was written against 2.5 and has long been out of my control. We sold the source to the sole client (was a financial services migration tool for a very specific domain during a joint venture; I can't divulge anymore).

I know longer work at the company, but i'd wager that the client wouldnt have been willing to pay $500/hr (the rate my company billed me out at for support, features, etc), 10 years ago, to have me port the app and the handful of 3rd party dependencies to Python 3.

Moving from 2 to 3 took a while. I moved once the libs I used moved. For an "app" developer, the migration was easy once my dependencies were ready. Most of the changes were straightforward. The print statement becoming a function was easy. str becoming Unicode and introducing bytes has been a headache. Still having issues time to time with text encoding, especially with encoded text dealing with SQL Server (looking at the default CP-1252 for US English). Another one that still trips me up is is needing a "newline=''" argument when opening as CSV file.

That said, I love Python 3. Ita mostly about forgetting/relearning old syntax, which is going to happen for anything youve been using for decades and undergoes a significant change. I started with Python over 15 years ago, at version 2.2, for reference.