Support newer python versions

[JoaoFelipe]

We need to update noworkflow to support newer Python version. I'm creating this issues to document what we need to fix on noworkflow for that.

[JoaoFelipe]

Before Python 3.9, Subscript was defined as:

Subscript(expr value, slice slice, expr_context ctx)

and slice was defined as

slice = Slice(expr? lower, expr? upper, expr? step)
          | ExtSlice(slice* dims)
          | Index(expr value)

Now, subscript accepts an expression as slice:

Subscript(expr value, expr slice, expr_context ctx)

The Index and ExtSlice nodes were removed, and Slice became an expression.
I don't think we have a special logic for Index in noWorkflow, so we don't need to change anything about that. But we may need to change what we had for ExtSlice and for Slice.