Mypy incorrectly warns when passing literals as kwarg keywords

**Bug Report**

```
from typing import Any, Dict, Literal
kw: Dict[Literal["a", "b"], Any]
def func(a, b):
    pass
func(**kw)
```
Leads to
```
main.py:8: error: Keywords must be strings
```

https://mypy-play.net/?mypy=latest&python=3.9&gist=582cf5e8c0a6e6a5881d391da64e1455

**Expected Behavior**
Expect this to be allowable - as these literals count as strings