input: Write a python function to group a dataframe by 'customer_id' and sum up the 'sales' column using pandas.
  assert group_and_sum_sales(pd.DataFrame({'customer_id': [1, 2, 1, 2, 3], 'sales': [10, 20, 30, 40, 50]})).equals(pd.Series({1: 40, 2: 60, 3: 50}, name='sales'))
  assert group_and_sum_sales(pd.DataFrame({'customer_id': [1, 1, 1], 'sales': [5, 10, 15]})).equals(pd.Series({1: 30}, name='sales'))
  assert group_and_sum_sales(pd.DataFrame({'customer_id': [1, 2, 3], 'sales': [100, 200, 300]})).equals(pd.Series({1: 100, 2: 200, 3: 300}, name='sales'))
output: def group_and_sum_sales(df):
      return df.groupby('customer_id')['sales'].sum()