fix _AtomicFile.close ignoring delete parameter

[bysiber]

_AtomicFile.close() accepts a delete parameter, and __exit__ passes delete=True when an exception occurs during the context manager block. However, the close method unconditionally calls os.replace, moving the (potentially incomplete) temp file over the original regardless of whether delete is True.

This defeats the purpose of atomic writes — if an error occurs while writing, the original file should be preserved and the temp file should be cleaned up instead.

This change makes close check the delete flag. When True, it removes the temp file with os.unlink rather than replacing the original. The os.unlink call is wrapped in a try/except to handle cases where the temp file may have already been removed.