**Apache Airflow version**: 2.1.0

**What happened**:

Using a Label infront of a TaskGroup breaks task dependencies, as tasks before the TaskGroup will now point to the last task in the TaskGroup.

**What you expected to happen**:

Task dependencies with or without Labels should be the same (and preferably the Label should be visible outside of the TaskGroup).
Behavior without a Label is as follows:

```python
with TaskGroup("tg1", tooltip="Tasks related to task group") as tg1:
        DummyOperator(task_id="b") >> DummyOperator(task_id="c")

DummyOperator(task_id="a") >> tg1
```
![without Taskgroup](https://user-images.githubusercontent.com/35238779/128502690-58b31577-91d7-4b5c-b348-4779c9286958.png)

**How to reproduce it**:

```python
with TaskGroup("tg1", tooltip="Tasks related to task group") as tg1:
        DummyOperator(task_id="b") >> DummyOperator(task_id="c")

    DummyOperator(task_id="a") >> Label("testlabel") >> tg1
```
![with TaskGroup](https://user-images.githubusercontent.com/35238779/128502758-5252b7e7-7c30-424a-ac3a-3f9a99b0a805.png)


"""Get a child task/TaskGroup by its label (i.e. task_id/group_id without the group_id prefix)"""
        return self.children[self.child_id(label)]
    def serialize_for_task_group(self) -> tuple[DagAttributeTypes, Any]:
        """Required by DAGNode."""
        from airflow.serialization.serialized_objects import TaskGroupSerialization
        return DagAttributeTypes.TASK_GROUP, TaskGroupSerialization.serialize_task_group(self)
    def topological_sort(self, _include_subdag_tasks: bool = False):
        """
        Sorts children in topographical order, such that a task comes after any of its
        upstream dependencies.
        :return: list of tasks in topological order
        """
        
        
        from airflow.operators.subdag import SubDagOperator  
        graph_unsorted = copy.copy(self.children)
        graph_sorted: list[DAGNode] = []
        
        if len(self.children) == 0:
            return graph_sorted
        
        while graph_unsorted: