Related to Issue #43594.

When running the following code

```
from abc import ABCMeta, ABC
from typing import Union

class MetaclassMixin(ABC):
    pass 

class Meta(MetaclassMixin, ABCMeta):
    pass

print(Union[str, Meta])
```

An exception is raised

 >>> TypeError: descriptor '__subclasses__' of 'type' object needs an argument


Tested on v3.6.9