
**Apache Airflow version**: 2.0.1


**Kubernetes version (if you are using kubernetes)** (use `kubectl version`): NA

**Environment**: Windows WSL2 (Ubuntu) Local

- **Cloud provider or hardware configuration**:
- **OS** (e.g. from /etc/os-release): Ubuntu 18.04
- **Kernel** (e.g. `uname -a`): Linux d255bce4dcd5 5.4.72-microsoft-standard-WSL2
- **Install tools**: Docker -compose
- **Others**:

**What happened**:
I have a dag with tasks:
Task1 - Get Date
Task 2 - Get data from Api call (Have set retires to 3)
Task 3 - Load Data

Task 2 had failed after three attempts. I am unable to clear the task Instance and get the below error in UI. 

[Dag Code](https://github.com/anilkulkarni87/airflow-docker/blob/master/dags/covidNyDaily.py)

```
Python version: 3.8.7
Airflow version: 2.0.1rc2
Node: d255bce4dcd5
-------------------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/airflow/.local/lib/python3.8/site-packages/flask/app.py", line 2447, in wsgi_app
    response = self.full_dispatch_request()
  File "/home/airflow/.local/lib/python3.8/site-packages/flask/app.py", line 1952, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/home/airflow/.local/lib/python3.8/site-packages/flask/app.py", line 1821, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/home/airflow/.local/lib/python3.8/site-packages/flask/_compat.py", line 39, in reraise
    raise value
  File "/home/airflow/.local/lib/python3.8/site-packages/flask/app.py", line 1950, in full_dispatch_request
    rv = self.dispatch_request()
  File "/home/airflow/.local/lib/python3.8/site-packages/flask/app.py", line 1936, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/home/airflow/.local/lib/python3.8/site-packages/airflow/www/auth.py", line 34, in decorated
    return func(*args, **kwargs)
  File "/home/airflow/.local/lib/python3.8/site-packages/airflow/www/decorators.py", line 60, in wrapper
    return f(*args, **kwargs)
  File "/home/airflow/.local/lib/python3.8/site-packages/airflow/www/views.py", line 1547, in clear
    return self._clear_dag_tis(
  File "/home/airflow/.local/lib/python3.8/site-packages/airflow/www/views.py", line 1475, in _clear_dag_tis
    count = dag.clear(
  File "/home/airflow/.local/lib/python3.8/site-packages/airflow/utils/session.py", line 65, in wrapper
    return func(*args, session=session, **kwargs)
  File "/home/airflow/.local/lib/python3.8/site-packages/airflow/models/dag.py", line 1324, in clear
    clear_task_instances(
  File "/home/airflow/.local/lib/python3.8/site-packages/airflow/models/taskinstance.py", line 160, in clear_task_instances
    ti.max_tries = ti.try_number + task_retries - 1
TypeError: unsupported operand type(s) for +: 'int' and 'str'
```

**What you expected to happen**:

I expected to clear the Task Instance so that the task could be scheduled again.

**How to reproduce it**:
1) Clone the repo link shared above
2) Follow instructions to setup cluster.
3) Change code to enforce error in Task 2 
4) Execute and try to clear task instance after three attempts.

![Error pops up when clicked on Clear](https://user-images.githubusercontent.com/10644132/107998258-8e1ee180-6f99-11eb-8442-0c0be5b23478.png)







tests/core/test_core.py
task = self.dag_bash.tasks[0]
        task.clear(start_date=DEFAULT_DATE, end_date=DEFAULT_DATE, upstream=True, downstream=True)
        ti = TaskInstance(task=task, execution_date=DEFAULT_DATE)
        ti.are_dependents_done()
    def test_illegal_args(self):
        """
        Tests that Operators reject illegal arguments
        """
        msg = 'Invalid arguments were passed to BashOperator (task_id: test_illegal_args).'
        with conf_vars({('operators', 'allow_illegal_arguments'): 'True'}):
            with pytest.warns(PendingDeprecationWarning) as warnings:
                BashOperator(
                    task_id='test_illegal_args',
                    bash_command='echo success',
                    dag=self.dag,
                    illegal_argument_1234='hello?',
                )
                assert any(msg in str(w) for w in warnings)
    def test_illegal_args_forbidden(self):