input: Write a function in Python that takes a list of objects and returns a list of those objects sorted by one of their attributes.
  
  def sort_by_attr(objects, attr):
   return sorted(objects, key=lambda item: item[attr])
  
  sort_by_attr([{"name": "Alice", "age": 20}, {"name": "Bob", "age": 21}], "age")
output: def sort_by_attr(objects, attr):
   return sorted(objects, key=lambda item: item[attr])