TypeScript -> Python creates a new synthetic type for unions of interfaces

See https://app.quicktype.io/?share=av9JMpZCKBG1DtHb7phv

```
interface Foo {
foo: string;
}

interface Bar {
bar: string;
}

interface Container {
op: Foo | Bar;
}
```

results in

```
from dataclasses import dataclass
from typing import Optional

@dataclass
class Foo:
foo: str

@dataclass
class Bar:
bar: str

@dataclass
class Op:
foo: Optional[str] = None
bar: Optional[str] = None

@dataclass
class Container:
op: Op
```

when ideally, it should result in
```
from dataclasses import dataclass
from typing import Optional, Union

@dataclass
class Foo:
foo: str

@dataclass
class Bar:
bar: str

@dataclass
class Container:
op: Union[Foo, Bar]
```