Issue description

replace_with function is typed as:

def replace_with(repl_str: str) -> ParseAction:

It is my understanding that replace_with it so be used with set_parse_action to return an arbitrary object as a parse result. Also, it's own docstring has replace_with(math.nan), with a non-string argument. Finally, set_parse_action accepts a Callable with return type Any.

The current implementation works, but produces errors when type-checking, e.g. with mypy:

error: Argument 1 has incompatible type "dict[Never, Never]"; expected "str"  [arg-type]

Proposed solution

I believe replace_with should have an argument of type Any.