input: Write a python function to return a list of uppercase letters from a given string.
  assert get_uppercase("Hello World") == ['H', 'W']
  assert get_uppercase("Python is FUN") == ['P', 'F', 'U', 'N']
  assert get_uppercase("no uppercase") == []
output: def get_uppercase(s):
      return [char for char in s if char.isupper()]