gh-119512: Use new REPL for the code module

[tanloong]

This adds an optional use_pyrepl: bool|None=False parameter to code.InteractiveConsole.interact() and code.interact().

When set as True, pyrepl is used. When False (the default), the basic readline-based REPL is used. When None, pyrepl is used if available and the PYTHON_BASIC_REPL environment variable is not set.

• Issue: Use new REPL for the "code" module #119512

[skirpichev]

CC @pablogsal, @ambv

[skirpichev]

Why not make None behavior - default, and get rid of the use_pyrepl option? Users can override this by setting PYTHON_BASIC_REPL.

I presume, new option was chosen on ground backward-compatibility, but new REPL is an incompatible change by itself, this is just a continuation.

So, I think it's fine to remove added option, but please wait first feedback from core developers.

[tanloong]

Thanks for your review!

I believe adding the use_pyrepl parameter is necessary, and it should default to False rather than None.

While the new REPL itself is a incompatible change, many third-party projects subclass code.InteractiveConsole to implement custom shells (1.2k search results on GitHub). CPython's own sqlite3 CLI also builds on code.InteractiveConsole and uses completely different syntax and completion logic from Python. Many of these 1.2k subclasses likely implement their own custom syntax and completions, just like the sqlite3 CLI.

If we removed the use_pyrepl parameter and enabled the new REPL by default, it would break those custom shells, both the official sqlite3 CLI and many third-party ones.

For backward compatibility, I think keeping use_pyrepl=False as the default is the safest approach.

[skirpichev]

This is unrelated change, please revert.