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.

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 relies on GNU Readline for completion. If we enable the new REPL by default, its completion will break, and we will also see incorrect syntax highlighting: tokens in SQLite statements that coincide with Python keywords will be highlighted mistakenly by PyREPL. Similar issues would affect those among the 1.2k subclasses that implemented their own custom syntax and completions.

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