Support Union types again

[SvenWarnke]

Consider the scenario, where an argument could have one of multiple types (see code below). Under version 0.2.1 this code worked. Now under 1.0.0 it breaks with "TypeError: Union types other than 'Optional' are not supported"

from dataclasses import dataclass, field
from typing import Union
from argparse_dataclass import ArgumentParser


def str_or_int(value: str) -> Union[str, int]:
    try:
        return int(value)
    except ValueError:
        return value


@dataclass
class Options:
    str_or_int: Union[str, int] = field(metadata=dict(type=str_or_int))


parser = ArgumentParser(Options)
print(parser.parse_args(["--str-or-int", "John"]))
print(parser.parse_args(["--str-or-int", "42"]))

I think this is a legitimate use case and by having the 'type' field in metadata the user can take up the burden of transforming those. We are currently using argparse-dataclass 0.2.1 with Union types so this change in behavior is preventing us from updating at the moment.

It would be great to have this support for Union because I would really like to update and get support for Optional types as well.

[mivade]

Closed via #42.