Fix dictionary mutation during pretty printing - #15474

Summary

Fixes a RuntimeError: dictionary changed size during iteration that can occur when IPython pretty-prints dictionaries whose contents change during formatting.

Problem

IPython's dictionary pretty-printer iterates over dictionary keys while formatting their associated values. In certain cases, formatting can trigger changes to the dictionary, invalidating the active iterator.

This can be reproduced using:

import posix
vars(posix)

Solution

Updated the dictionary pretty-printer in IPython/lib/pretty.py to iterate over a snapshot of dictionary keys rather than a live dictionary view.

This prevents changes to dictionary size from invalidating the key iterator during formatting.

Tests

Added regression tests covering:

• Dictionary mutation during pretty-printing.
• Standard dictionary formatting.
• Recursive dictionary formatting.
• Pretty-printing the posix module dictionary.