gh-153506: Fix IDLE crash on non-iterable __all__

[muhamedfazalps]

Problem

Fixes gh-153506

When a module has __all__ set to a non-iterable value (like None or an integer), pressing Tab for module-name completion in IDLE's shell raises an uncaught TypeError.

Root Cause

autocomplete.py calls sorted(eval("__all__", namespace)) without checking if the value is iterable. If __all__ = None (an easy typo), sorted() raises TypeError.

Solution

Add a hasattr(__all__, '__iter__') check before calling sorted(). If __all__ is not iterable, fall back to the standard private-name filtering ([s for s in bigl if s[:1] != '_']).

This is consistent with how Python's import system handles non-iterable __all__ — it raises an error, but IDLE should be resilient and fall back gracefully.

Testing

Verified that:

1. __all__ = None no longer crashes — falls back to standard completion
2. __all__ = ['valid', 'names'] still works correctly
3. Normal completion without __all__ still works

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[StanFromIreland]

Closing in favour of existing PR: #153507