Import Literal from typing now that 3.7 dropped

[ascillitoe]

Now that we support Python >= 3.8 (#825) we could do:

from typing import Literal

instead of:

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...