text: Title: Error when extracting upstream references
  Body: Happens when using the getitem operator with slices (e.g. `df.loc[:10, 'a']`)
  
  ```pytb
    File "/Users/Edu/miniconda3/envs/package_name/lib/python3.7/site-packages/ploomber/static_analysis/python.py", line 38, in <setcomp>
      if isinstance(node, ast.Subscript) and node.value.id == 'upstream'
  AttributeError: 'Attribute' object has no attribute 'id'
  ```