input: Write a python function to convert a string into both uppercase and lowercase.
  assert string_case('Hello, World!') == ('HELLO, WORLD!', 'hello, world!')
  assert string_case('Python Programming') == ('PYTHON PROGRAMMING', 'python programming')
  assert string_case('123 ABC xyz') == ('123 ABC XYZ', '123 abc xyz')
output: def string_case(s):
      return s.upper(), s.lower()