input: Write a python function to read a CSV file and return the sum of each column.
  assert read_col_sums('test1.csv') == [15, 25, 35]
  assert read_col_sums('test2.csv') == [30, 45, 60]
  assert read_col_sums('test3.csv') == [7, 14, 21]
output: def read_col_sums(filename):
      import csv
      with open(filename, mode='r') as file:
          csv_reader = csv.reader(file)
          sums = []
          for row in csv_reader:
              if not sums:
                  sums = [0] * len(row)
              for i, value in enumerate(row):
                  sums[i] += int(value)
      return sums