Import Literal type from typing_extensions if python version<3.8

[hukkin]

typing.Literal was added in Python 3.8, so type checkers will fail trying to import it if python_version<3.8

This PR does the following:

1. Imports Literal from typing_extensions if python_version<3.8.
2. Adds typing_extensions as an install requirement if python_version<3.8.

Not sure if we want 2. though. One alternative would be to remove the one instance of Literal[False] that we have in the .pyi file.

[1st1]

Not sure if we want 2. though. One alternative would be to remove the one instance of Literal[False] that we have in the .pyi file.

I'd remove the use of Literal if possible to simplify things.

[hukkin]

👍 Removed use of Literal