**Apache Airflow version**: 2.0.1

**What happened**:

The operator fails at the last step, after successfully refreshing the workbook with this error:
```
tableauserverclient.server.endpoint.exceptions.ServerResponseError: 

        401002: Unauthorized Access
                Invalid authentication credentials were provided.
```

**What you expected to happen**:
It should not fail, like when we use the username/password authentication method (instead of personal_access_token)

<!-- What do you think went wrong? -->

Tableau server does not allow concurrent connections when using personal_access_token https://github.com/tableau/server-client-python/issues/717
The solution would be redesigning completely the operator to only call the hook once.
My quick fix was to edit this in TableauHook:

```
    def __exit__(self, exc_type: Any, exc_val: Any, exc_tb: Any) -> None:
        pass
```

**How to reproduce it**:

Run this operator TableauRefreshWorkbookOperator using Tableau personal_access_token authentication (token_name, personal_access_token).

airflow/providers/tableau/hooks/tableau.py
super().__init__()
        self.tableau_conn_id = tableau_conn_id
        self.conn = self.get_connection(self.tableau_conn_id)
        self.site_id = site_id or self.conn.extra_dejson.get('site_id', '')
        self.server = Server(self.conn.host)
        verify = self.conn.extra_dejson.get('verify', 'True')
        try:
            verify = bool(strtobool(verify))
        except ValueError:
            pass
        self.server.add_http_options(
            options_dict={'verify': verify, 'cert': self.conn.extra_dejson.get('cert', None)}
        )
        self.server.use_server_version()
        self.tableau_conn = None
    def __enter__(self):