My favourite part:

"That could work; it's a bit late to do this in 3.2, though. How about the following transition strategy for the complex output.

Python 3.3: Introduce PYTHONIMAGINARYSYMBOL environment variable (and possibly also a related command-line option to the interpreter?).

Python 3.4: Show a warning on startup if this environment variable isn't used.

Python 3.5: Make the environment variable mandatory.

Python 3.6: Make the environment variable optional again, but this time with the default output being 'i' rather than 'j'.

Python 3.7: Deprecate use of PYTHONIMAGINARYSYMBOL. (Warning on startup if it's set.)

Python 3.8: Error on startup if PYTHONIMAGINARYSYMBOL is set.

Python 3.9: Go back to ignoring PYTHONIMAGINARYSYMBOL."