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**Apache Airflow version**: 2.1.0


**Kubernetes version (if you are using kubernetes)** (use `kubectl version`):1.21

**Environment**: 

- **Cloud provider or hardware configuration**:
- **OS** (e.g. from /etc/os-release): 
- **Kernel** (e.g. `uname -a`):Linux workspace
- **Install tools**:
- **Others**:

**What happened**:

Running PostgresOperator errors out with  'PostgresHook' object has no attribute 'schema'.
I tested it as well with the code from PostgresOperator tutorial in https://airflow.apache.org/docs/apache-airflow-providers-postgres/stable/operators/postgres_operator_howto_guide.html 

This is happening since upgrade of apache-airflow-providers-postgres to version 2.1.0

```
*** Reading local file: /tmp/logs/postgres_operator_dag/create_pet_table/2021-08-04T15:32:40.520243+00:00/2.log
[2021-08-04 15:57:12,429] {taskinstance.py:876} INFO - Dependencies all met for <TaskInstance: postgres_operator_dag.create_pet_table 2021-08-04T15:32:40.520243+00:00 [queued]>
[2021-08-04 15:57:12,440] {taskinstance.py:876} INFO - Dependencies all met for <TaskInstance: postgres_operator_dag.create_pet_table 2021-08-04T15:32:40.520243+00:00 [queued]>
[2021-08-04 15:57:12,440] {taskinstance.py:1067} INFO - 
--------------------------------------------------------------------------------
[2021-08-04 15:57:12,440] {taskinstance.py:1068} INFO - Starting attempt 2 of 2
[2021-08-04 15:57:12,440] {taskinstance.py:1069} INFO - 
--------------------------------------------------------------------------------
[2021-08-04 15:57:12,457] {taskinstance.py:1087} INFO - Executing <Task(PostgresOperator): create_pet_table> on 2021-08-04T15:32:40.520243+00:00
[2021-08-04 15:57:12,461] {standard_task_runner.py:52} INFO - Started process 4692 to run task
[2021-08-04 15:57:12,466] {standard_task_runner.py:76} INFO - Running: ['airflow', 'tasks', 'run', '***_operator_dag', 'create_pet_table', '2021-08-04T15:32:40.520243+00:00', '--job-id', '6', '--pool', 'default_pool', '--raw', '--subdir', 'DAGS_FOLDER/test_dag.py', '--cfg-path', '/tmp/tmp2mez286k', '--error-file', '/tmp/tmpgvc_s17j']
[2021-08-04 15:57:12,468] {standard_task_runner.py:77} INFO - Job 6: Subtask create_pet_table
[2021-08-04 15:57:12,520] {logging_mixin.py:104} INFO - Running <TaskInstance: ***_operator_dag.create_pet_table 2021-08-04T15:32:40.520243+00:00 [running]> on host 5995a11eafd1
[2021-08-04 15:57:12,591] {taskinstance.py:1280} INFO - Exporting the following env vars:
AIRFLOW_CTX_DAG_OWNER=airflow
AIRFLOW_CTX_DAG_ID=***_operator_dag
AIRFLOW_CTX_TASK_ID=create_pet_table
AIRFLOW_CTX_EXECUTION_DATE=2021-08-04T15:32:40.520243+00:00
AIRFLOW_CTX_DAG_RUN_ID=manual__2021-08-04T15:32:40.520243+00:00
[2021-08-04 15:57:12,591] {postgres.py:68} INFO - Executing: 
            CREATE TABLE IF NOT EXISTS pet (
            pet_id SERIAL PRIMARY KEY,
            name VARCHAR NOT NULL,
            pet_type VARCHAR NOT NULL,
            birth_date DATE NOT NULL,
            OWNER VARCHAR NOT NULL);
          
[2021-08-04 15:57:12,608] {base.py:69} INFO - Using connection to: id: ***_default.
[2021-08-04 15:57:12,610] {taskinstance.py:1481} ERROR - Task failed with exception
Traceback (most recent call last):
  File "/usr/local/lib/python3.8/site-packages/airflow/models/taskinstance.py", line 1137, in _run_raw_task
    self._prepare_and_execute_task_with_callbacks(context, task)
  File "/usr/local/lib/python3.8/site-packages/airflow/models/taskinstance.py", line 1311, in _prepare_and_execute_task_with_callbacks
    result = self._execute_task(context, task_copy)
  File "/usr/local/lib/python3.8/site-packages/airflow/models/taskinstance.py", line 1341, in _execute_task
    result = task_copy.execute(context=context)
  File "/usr/local/lib/python3.8/site-packages/airflow/providers/postgres/operators/postgres.py", line 70, in execute
    self.hook.run(self.sql, self.autocommit, parameters=self.parameters)
  File "/usr/local/lib/python3.8/site-packages/airflow/hooks/dbapi.py", line 177, in run
    with closing(self.get_conn()) as conn:
  File "/usr/local/lib/python3.8/site-packages/airflow/providers/postgres/hooks/postgres.py", line 97, in get_conn
    dbname=self.schema or conn.schema,
AttributeError: 'PostgresHook' object has no attribute 'schema'
[2021-08-04 15:57:12,612] {taskinstance.py:1524} INFO - Marking task as FAILED. dag_id=***_operator_dag, task_id=create_pet_table, execution_date=20210804T153240, start_date=20210804T155712, end_date=20210804T155712
[2021-08-04 15:57:12,677] {local_task_job.py:151} INFO - Task exited with return code 1
```

**What you expected to happen**:

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

**How to reproduce it**:
<!---

As minimally and precisely as possible. Keep in mind we do not have access to your cluster or dags.

If you are using kubernetes, please attempt to recreate the issue using minikube or kind.

## Install minikube/kind

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**Anything else we need to know**:

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airflow/providers/postgres/hooks/postgres.py
"""Establishes a connection to a postgres database."""
        conn_id = getattr(self, self.conn_name_attr)
        conn = deepcopy(self.connection or self.get_connection(conn_id))
        
        if conn.extra_dejson.get('iam', False):
            conn.login, conn.password, conn.port = self.get_iam_token(conn)
        conn_args = dict(
            host=conn.host,
            user=conn.login,
            password=conn.password,
            dbname=self.schema or conn.schema,
            port=conn.port,
        )
        raw_cursor = conn.extra_dejson.get('cursor', False)
        if raw_cursor:
            conn_args['cursor_factory'] = self._get_cursor(raw_cursor)
        for arg_name, arg_val in conn.extra_dejson.items():
            if arg_name not in [
                'iam',
                'redshift',
                'cursor',
                'cluster-identifier',
                'aws_conn_id',
            ]:
                conn_args[arg_name] = arg_val
        self.conn = psycopg2.connect(**conn_args)
        return self.conn
    def copy_expert(self, sql: str, filename: str) -> None: