input: Write a python function to extract unique elements from a list.
  assert extract_unique([1, 1, 2, 3, 4, 5, 5]) == {1, 2, 3, 4, 5}
  assert extract_unique([10, 20, 20, 30, 40, 40, 50]) == {10, 20, 30, 40, 50}
  assert extract_unique([5, 7, 9, 7, 6, 5, 3]) == {3, 5, 6, 7, 9}
output: def extract_unique(lst):
      return set(lst)