I didn't get where I am today without being part of the "Statler and Waldorf" wing of El Reg. But my medication is working well today, so I will play-nice and share my top two tips for migrating production Python sites from 2.x to 3.x
1) Replace all PRINT with a wrapper function (say) PrintThis()
PrintThis() can then include a check on the config Development/Production status and switch accordingly.
Print to console is fine in Dev. But in Production, PrintThis() should switch output to Log files or a Log database (your choice)
2) For every INCLUDEd library in your 2.x code
sudo python3 pip -m install ThisLibrary
On Windows (e.g. in Visual Studio):
Switch the Python Environment to 3.x.
Use pip and search PyPI for each library, and install.