Fix sendspin serve crash on Python 3.12 (#252)

`sendspin serve` raised `TypeError: type 'Synchronized' is not
subscriptable` at import time on Python 3.12 because
`multiprocessing.sharedctypes.Synchronized[int]` only gained
`__class_getitem__` in 3.13.

Author: maximmaxim345

sendspin/serve/server.py

@@ -1,5 +1,7 @@
 """Custom SendspinServer with embedded web player."""
 
+from __future__ import annotations
+
 from importlib.resources import files
 from multiprocessing.sharedctypes import Synchronized
 from pathlib import Path