refactor.typing: conditionally import Self for compatibility

- Remove direct import of Self from typing for Python versions < 3.11
- Keep the import from typing_extensions for older Python versions

Author: tanbro

src/redis_func_cache/typing.py

@@ -9,7 +9,7 @@
     from typing import TypeGuard
 
 if TYPE_CHECKING:  # pragma: no cover
-    from typing import Protocol, Self
+    from typing import Protocol
 
     if sys.version_info < (3, 11):  # pragma: no cover
         from typing_extensions import Self