Silence tokenization errors in ultratb. Currently, the ultratb module attempts to tokenize the code it finds in tracebacks, and issues a scary-sounding warnings about potentially invalid tracebacks if it fails. However, this tokenization is known to fail in some cases, notably if the reported error line is the terminating line of a multiline, triple-quoted string literal. This change fixes the issue by switching this to a `logging.debug` message, which silences it by default without completely removing it for debugging purposes. Fixes #6864.