input: Write a function to remove all duplicate elements from a given list of integers. If the given list is empty then return an empty list.
  assert remove_duplicates([1, 2, 3, 2, 4, 5, 6, 3])==[1, 2, 3, 4, 5, 6]
  assert remove_duplicates([5, 3, 4, 1, 2, 2, 5, 3, 1])==[5, 3, 4, 1, 2]
  assert remove_duplicates([7, 7, 7, 7])==[7]
  assert remove_duplicates([])==[]
output: def remove_duplicates(lst):
      return list(dict.fromkeys(lst))