IDLE fails to start or raises when an editor window's file does not exist

Bug report

Bug description:

IDLE raises an uncaught traceback when a file that is open in the editor is deleted or renamed by another program and the IDLE window is then refocused.

EditorWindow.focus_in_event runs on every <FocusIn> event and calls EditorWindow.last_mtime, which calls os.path.getmtime(self.io.filename) with no error handling. When the open file has been removed or renamed, getmtime raises FileNotFoundError, which escapes the Tk event handler and breaks the modified-on-disk reload check.

Reproduction

Open a file in the IDLE editor, delete or rename it in another program, then click back into the IDLE window. A traceback like this reaches the console and the reload check stops working:

FileNotFoundError: [Errno 2] No such file or directory: '.../somefile.py'

The same call reproduced without a GUI:

import os, tempfile, types
from idlelib.editor import EditorWindow

with tempfile.TemporaryDirectory() as d:
    path = os.path.join(d, 'gone.py')
    open(path, 'w').close()
    mtime = os.path.getmtime(path)
    os.remove(path)
    stub = types.SimpleNamespace(io=types.SimpleNamespace(filename=path), mtime=mtime)
    EditorWindow.last_mtime(stub)   # raises FileNotFoundError on the main branch

Suggested fix

Guard os.path.getmtime in last_mtime with try/except OSError and return the last known mtime, so focus_in_event sees no change and does not prompt to reload a file that is gone.

CPython versions tested on:

3.16, CPython main branch

Operating systems tested on:

macOS

Linked PRs

• gh-153481

Start in idlelib.editor at EditorWindow.last_mtime and its caller, focus_in_event. Reproduce the missing-file case from the issue, then verify that refocusing IDLE no longer raises and that the reload check does not prompt for a deleted file.