Hello,

For following code:

```
import dataclasses
import typing
from datetime import datetime


@dataclasses.dataclass
class Foo:
    datetime: typing.Optional[datetime] = None


print(dataclasses.fields(Foo)[0].type)
```

`datetime` `Foo` attribute have `NoneType` type. Problem come from `datetime` attribute name is already used by the `from datetime import datetime` import. I'm not sure if it is a bug or a strange behavior but it seems not regular. Tested on python 3.8.0a2 with same result.