input: Write a function to filter positive numbers using lambda function.
  assert positive([-10, -5, 0, 5, 10]) == [5, 10]
  assert positive([-1, 1, -2, 2, -3, 3]) == [1, 2, 3]
  assert positive([-20, -10, -5, -1]) == []
output: def positive(numbers):
      return list(filter(lambda x: x > 0, numbers))