input: Write a function to convert a given string to its opposite case. The function should swap all lowercase characters to uppercase and vice versa.
  
  def convert_string_case(string):
      new_string = string.swapcase()
      return new_string
  
  assert convert_string_case("this Is A TeST")=="THIS iS a tEst"
  assert convert_string_case("HELLO world")=="hello WORLD"
  assert convert_string_case("Python3.8")=="pYTHON3.8"
  assert convert_string_case("")==""
  assert convert_string_case("123")=="123"
output: def convert_string_case(string):
      new_string = string.swapcase()
      return new_string