Python slice notation is not parsed

[schnoeggi]

i recognized that Boa does not correctly parse the python slice notation.

For example
label = file_name.split('/')[:-2] will be parsed to label = file_name.split('/')[-2]

When looking at the AST of the expression i see that the colon is not captured:

{
....
            {
               "kind": "ARRAYINDEX",
               "expressions": [
                  {
                     "kind": "UNARY",
                     "expressions": [
                        {
                           "kind": "LITERAL",
                           "literal": "-"
                        },
                        {
                           "kind": "LITERAL",
                           "literal": "2"
                        }
                     ]
                  }
            ...
   ]
}