input: Write a function to filter strings by length.
  assert filter_by_length(['ada', 'python', 'ruby', 'egg', 'coffee']) == ['ada', 'egg']
  assert filter_by_length(['cat', 'dog', 'mouse', 'elephant', 'bat']) == ['cat', 'dog', 'bat']
  assert filter_by_length(['java', 'c', 'cpp', 'html', 'css']) == ['cpp', 'css']
output: def filter_by_length(strings):
      return [s for s in strings if len(s) <= 3]