Remove old 3.8 code

[gordonwatts]

This code can probably now be removed:

                # This if statement can be removed when we no longer support 3.8.
                if isinstance(slice, ast.Index):
                    slice = slice.value  # type: ignore
                if not isinstance(slice, ast.Constant):
                    raise ValueError(
                        f"Slices must be indexable constants only - {ast.dump(slice)} is not "
                        "valid."
                    )
                index = slice.value