input: Write a python function to convert a list of strings to uppercase using list comprehension.
  assert to_uppercase(['hello', 'world']) == ['HELLO', 'WORLD']
  assert to_uppercase(['python', 'is', 'fun']) == ['PYTHON', 'IS', 'FUN']
  assert to_uppercase(['a', 'b', 'c']) == ['A', 'B', 'C']
output: def to_uppercase(strings):
      return [s.upper() for s in strings]