gh-104231: emit warning on __bytes__ and __str__ when returning strict subclass

[sunmy2019]

Returning strict subclasses at __str__ can cause unexpected behavior, since other parts of the VM have always assumed str(x) to be an instance of str, not its strict subclass. The same applies to the bytes(x).

Hence, we deprecated the ability to return strict subclasses of str/bytes at __str__/__bytes__, respectively.

• Issue: Superfluous call to __init__ on str(x)/bytes(x) when __new__ returns an instance of str/bytes' subclass #104231

[github-actions]

This PR is stale because it has been open for 30 days with no activity.