intent: Pandas sum by groupby, but exclude certain columns
rewritten_intent: Get sum of values of columns  'Y1961', 'Y1962', 'Y1963' after group by on columns "Country" and "Item_code" in dataframe `df`.
snippet: df.groupby(['Country', 'Item_Code'])[['Y1961', 'Y1962', 'Y1963']].sum()