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.
  
  ### Input:
  [{"name": "Alice", "age": 20}, {"name": "Bob", "age": 21}]
  
  ### Output:
  def sort_by_attr(objects, attr):
   return sorted(objects, key=lambda item: item[attr])
   
  assert sort_by_attr([{"name": "Alice", "age": 20}, {"name": "Bob", "age": 21}], "age") == [{"name": "Alice", "age": 20}, {"name": "Bob", "age": 21}]
  assert sort_by_attr([{"name": "Alice", "age": 21}, {"name": "Bob", "age": 20}], "age") == [{"name": "Bob", "age": 20}, {"name": "Alice", "age": 21}]
  assert sort_by_attr([{"name": "Charlie", "age": 22}], "age") == [{"name": "Charlie", "age": 22}]
  assert sort_by_attr([], "age") == []
output: def sort_by_attr(objects, attr):
   return sorted(objects, key=lambda item: item[attr])