simplify typing

src/textual/_asyncio.py

@@ -16,7 +16,7 @@ if sys.version_info >= (3, 8):
 else:
     import asyncio
     from asyncio import create_task as _create_task
-    from typing_extensions import Awaitable
+    from typing import Awaitable
 
     def create_task(coroutine: Awaitable, *, name: str | None = None) -> asyncio.Task:
         """Schedule the execution of a coroutine object in a spawn task."""

src/textual/_types.py

@@ -1,9 +1,8 @@
-from typing import Callable, List, Union
+from typing import TYPE_CHECKING, Awaitable, Callable, List, Union
-from typing import Awaitable, TYPE_CHECKING
-
 
 from typing_extensions import Protocol
 
+from rich.segment import Segment
 
 if TYPE_CHECKING:
     from .message import Message