Brace-based template arguments crash literal parsing with TypeError #703

parser.DefaultParseValue('{{name}}') raises TypeError: unhashable type: 'set' on current master. The same happens for {[1]: 2} and {[1]}. These inputs should fall back to the original string, as other values that cannot be parsed as literals do. This currently breaks CLI arguments containing brace-based templates.

I plan to handle TypeError alongside the existing literal-parsing exceptions and add regression coverage, while preserving valid dict/set parsing.