Import `Literal` from `typing` now that 3.7 dropped

Now that we support Python >= 3.8 (#825) we could do:

```python
from typing import Literal
```

instead of:

```python
from typing_extensions import Literal
```

Although, this depends if we want to keep unofficial support for 3.7 for a while. I vote `no`, but probably worth at least checking nothing else on our stack is locked into 3.7...