**Apache Airflow version**

13faa6912f7cd927737a1dc15630d3bbaf2f5d4d

**Environment**

- **Configuration**: Local Executor
- **OS** (e.g. from /etc/os-release): Mac OS 11.3
- **Kernel**: Darwin Kernel Version 20.4.0
- **Install tools**: `pip install -e .`

**The DAG**

```python
def callable():
    print("hi")

with DAG(dag_id="two_virtualenv") as dag:

    a = PythonOperator(
        task_id="a",
        python_callable=callable,
    )

    # b = PythonOperator(          # works
    b = PythonVirtualenvOperator(  # doesn't work
        task_id="b",
        python_callable=callable,
    )

    a >> b
```

**What happened**:
Failure somewhere between first task and second:
```
INFO - Marking task as SUCCESS. dag_id=two_virtualenv, task_id=a
ERROR - Failed to execute task: cannot pickle 'module' object.
Traceback (most recent call last):
  File "/Users/matt/src/airflow/airflow/executors/debug_executor.py", line 79, in _run_task
    ti._run_raw_task(job_id=ti.job_id, **params)  # pylint: disable=protected-access
  File "/Users/matt/src/airflow/airflow/utils/session.py", line 70, in wrapper
    return func(*args, session=session, **kwargs)
  File "/Users/matt/src/airflow/airflow/models/taskinstance.py", line 1201, in _run_raw_task
    self._run_mini_scheduler_on_child_tasks(session)
  File "/Users/matt/src/airflow/airflow/utils/session.py", line 67, in wrapper
    return func(*args, **kwargs)
  File "/Users/matt/src/airflow/airflow/models/taskinstance.py", line 1223, in _run_mini_scheduler_on_child_tasks
    partial_dag = self.task.dag.partial_subset(
  File "/Users/matt/src/airflow/airflow/models/dag.py", line 1490, in partial_subset
    dag.task_dict = {
  File "/Users/matt/src/airflow/airflow/models/dag.py", line 1491, in <dictcomp>
    t.task_id: copy.deepcopy(t, {id(t.dag): dag})  # type: ignore
  File "/usr/local/Cellar/python@3.9/3.9.4/Frameworks/Python.framework/Versions/3.9/lib/python3.9/copy.py", line 153, in deepcopy
    y = copier(memo)
  File "/Users/matt/src/airflow/airflow/models/baseoperator.py", line 961, in __deepcopy__
    setattr(result, k, copy.deepcopy(v, memo))  # noqa
  File "/usr/local/Cellar/python@3.9/3.9.4/Frameworks/Python.framework/Versions/3.9/lib/python3.9/copy.py", line 161, in deepcopy
    rv = reductor(4)
TypeError: cannot pickle 'module' object
ERROR - Task instance <TaskInstance: two_virtualenv.a 2021-05-11 00:00:00+00:00 [failed]> failed
```

**What you expected to happen**:

Both tasks say "hi" and succeed

**To Replicate**

The DAG and output above are shortened for brevity.  A more complete story: https://gist.github.com/MatrixManAtYrService/6b27378776470491eb20b60e01cfb675

Ran it like this:
```
 $ airflow dags test two_virtualenv $(date "+%Y-%m-%d")
```
tests/operators/test_python.py
start_date=timezone.utcnow(),
            execution_date=DEFAULT_DATE,
            state=State.RUNNING,
        )
        short_op.run(start_date=DEFAULT_DATE, end_date=DEFAULT_DATE)
        downstream.run(start_date=DEFAULT_DATE, end_date=DEFAULT_DATE)
        tis = dr.get_task_instances()
        for ti in tis:
            if ti.task_id == 'make_choice':
                assert ti.state == State.SUCCESS
            elif ti.task_id == 'downstream':
                assert ti.state == State.SKIPPED
            else:
                raise ValueError(f'Invalid task id {ti.task_id} found!')
        
        with create_session() as session:
            clear_task_instances([t for t in tis if t.task_id == "downstream"], session=session, dag=dag)
        
        downstream.run(start_date=DEFAULT_DATE, end_date=DEFAULT_DATE)
        
        for ti in dr.get_task_instances():
            if ti.task_id == 'make_choice':
                assert ti.state == State.SUCCESS
            elif ti.task_id == 'downstream':
                assert ti.state == State.SKIPPED
            else:
                raise ValueError(f'Invalid task id {ti.task_id} found!')
virtualenv_string_args: List[str] = []
class TestPythonVirtualenvOperator(unittest.TestCase):
    def setUp(self):