types: Fix missing runtime dependency

uv run --no-dev -p python3.13 -- python -c '
  from tmuxp import _internal, cli, workspace, exc, log, plugin, shell, types, util, __version__
  from tmuxp._internal import config_reader, types
  from tmuxp.workspace import builder, constants, finders, freezer, importers, loader, validation
  from libtmux import __version__ as __libtmux_version__
  print("tmuxp version:", __version__)
  print("libtmux version:", __libtmux_version__)
  '
  shell: /usr/bin/bash -e {0}
  env:
    UV_CACHE_DIR: /home/runner/work/_temp/setup-uv-cache
Using CPython 3.13.2
Creating virtual environment at: .venv
   Building tmuxp @ file:///home/runner/work/tmuxp/tmuxp
      Built tmuxp @ file:///home/runner/work/tmuxp/tmuxp
Installed 4 packages in 1ms
Traceback (most recent call last):
  File "<string>", line 3, in <module>
    from tmuxp._internal import config_reader, types
  File "/home/runner/work/tmuxp/tmuxp/src/tmuxp/_internal/types.py", line 15, in <module>
    from typing_extensions import NotRequired, TypedDict
ModuleNotFoundError: No module named 'typing_extensions'

Author: tony

src/tmuxp/_internal/types.py

@@ -12,7 +12,16 @@
 
 from __future__ import annotations
 
-from typing_extensions import NotRequired, TypedDict
+import typing as t
+from typing import TypedDict
+
+if t.TYPE_CHECKING:
+    import sys
+
+    if sys.version_info >= (3, 11):
+        from typing import NotRequired
+    else:
+        from typing_extensions import NotRequired
 
 
 class PluginConfigSchema(TypedDict):