Fix inspect.formatargvalues() to handle arguments that are no longer bound in the frame locals (e.g. after del) by showing them as <deleted> instead of raising KeyError.

Changes

• Added a _DELETED sentinel object with repr of <deleted> in Lib/inspect.py
• Changed formatargvalues() to use locals.get(name, _DELETED) instead of locals[name] for regular arguments, *args, and **kwargs
• Added two tests: one for deleted regular arguments, one for deleted *args/**kwargs

Reproducer (before fix)

import inspect

def fun(x):
    del x
    return inspect.currentframe()

inspect.formatargvalues(*inspect.getargvalues(fun(10)))
# KeyError: 'x'

After fix

inspect.formatargvalues(*inspect.getargvalues(fun(10)))
# '(x=<deleted>)'

• Issue: inspect.getargvalues fails if arg name is not bound to a value #61448

Note: This supersedes the stale PR #101318 which has been awaiting changes since August 2023 with merge conflicts.