Fix flake8 error in reawaitable.py

Add type cast to anyio.Lock to prevent name redefinition error while preserving type safety

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Co-Authored-By: Claude <noreply@anthropic.com>

Author: proboscis

returns/primitives/reawaitable.py

@@ -22,7 +22,7 @@ async def __aexit__(self, exc_type, exc_val, exc_tb) -> None: ...
 
     Lock: type[AsyncLock] = asyncio.Lock
 else:
-    Lock = anyio.Lock
+    Lock = cast(type[AsyncLock], anyio.Lock)
 
 _ValueType = TypeVar('_ValueType')
 _AwaitableT = TypeVar('_AwaitableT', bound=Awaitable)