Fix flyweight_with_metaclass sharing instances for different arguments (#495)
The pool key concatenated str(arg) with no separator, so Card2('1', '0') and
Card2('10') (or Card2(1) and Card2('1')) got the same key and the second call
returned the first call's instance. Use repr of the class name, args and
sorted kwargs instead, which also makes the key independent of kwarg order.
Add tests, since the module had none.
Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>