GH-94398: TaskGroup: Fail create_task() during shutdown (GH-94400)

Once the task group is shutting down, it should not be possible to create a new task.
Here "shutting down" means `self._aborting` is set, indicating that at least one task
has failed and we have cancelled all others.

Co-authored-by: Łukasz Langa <lukasz@langa.pl>

Lib/test/test_asyncio/test_taskgroups.py

@@ -122,10 +122,8 @@ class TestTaskGroup(unittest.IsolatedAsyncioTestCase):
         self.assertTrue(t2.cancelled())
 
     async def test_cancel_children_on_child_error(self):
-        """
-        When a child task raises an error, the rest of the children
-        are cancelled and the errors are gathered into an EG.
-        """
+        # When a child task raises an error, the rest of the children
+        # are cancelled and the errors are gathered into an EG.
 
         NUM = 0
         t2_cancel = False
@@ -722,6 +720,27 @@ class TestTaskGroup(unittest.IsolatedAsyncioTestCase):
             await t2
             self.assertEqual(2, ctx.get(cvar))
 
+    async def test_taskgroup_no_create_task_after_failure(self):
+        async def coro1():
+            await asyncio.sleep(0.001)
+            1 / 0
+        async def coro2(g):
+            try:
+                await asyncio.sleep(1)
+            except asyncio.CancelledError:
+                with self.assertRaises(RuntimeError):
+                    g.create_task(c1 := coro1())
+                # We still have to await c1 to avoid a warning
+                with self.assertRaises(ZeroDivisionError):
+                    await c1
+
+        with self.assertRaises(ExceptionGroup) as cm:
+            async with taskgroups.TaskGroup() as g:
+                g.create_task(coro1())
+                g.create_task(coro2(g))
+
+        self.assertEqual(get_error_types(cm.exception), {ZeroDivisionError})
+
 
 if __name__ == "__main__":
     unittest.main()