input: Write a python function to convert a string to uppercase without using the built-in upper() method.
  assert upper_string('hello') == 'HELLO'
  assert upper_string('Python') == 'PYTHON'
  assert upper_string('123abc') == '123ABC'
output: def upper_string(s):
      result = ''
      for char in s:
          if 'a' <= char <= 'z':
              result += chr(ord(char) - 32)
          else:
              result += char
      return result