input: Write a python function to check if a number is abundant.
  assert is_abundant(12) == True
  assert is_abundant(28) == False
  assert is_abundant(18) == True
  assert is_abundant(20) == True
  assert is_abundant(6) == False
output: def is_abundant(n):
      divisors_sum = sum(i for i in range(1, n) if n % i == 0)
      return divisors_sum > n