fix(snapshot): drop runtime typing_extensions import

why: CI matrix lacks typing_extensions in production deps; only Self is
needed for typing, which 3.11+ provides natively.
what:
- Move Self import under TYPE_CHECKING + sys.version_info gate
- Pattern matches existing libtmux/pane.py and window.py

Author: tony

src/libtmux/snapshot.py

@@ -5,18 +5,22 @@
 import dataclasses
 import datetime
 import json
+import sys
 import typing as t
 from abc import ABC, abstractmethod
 
-from typing_extensions import Self
-
 from libtmux.formats import PANE_FORMATS
 
 if t.TYPE_CHECKING:
     from collections.abc import Iterator, Sequence
 
     from libtmux.pane import Pane
 
+    if sys.version_info >= (3, 11):
+        from typing import Self
+    else:
+        from typing_extensions import Self
+
 
 class SnapshotOutputAdapter(ABC):
     """Base class for snapshot output adapters.