**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")
```
airflow/operators/python.py
The function must be defined using def, and not be
    part of a class. All imports must happen inside the function
    and no variables outside of the scope may be referenced. A global scope
    variable named virtualenv_string_args will be available (populated by
    string_args). In addition, one can pass stuff through op_args and op_kwargs, and one
    can use a return value.
    Note that if your virtualenv runs in a different Python major version than Airflow,
    you cannot use return values, op_args, op_kwargs, or use any macros that are being provided to
    Airflow through plugins. You can use string_args though.
    .. seealso::
        For more information on how to use this operator, take a look at the guide:
        :ref:`howto/operator:PythonVirtualenvOperator`
    :param python_callable: A python function with no references to outside variables,
        defined with def, which will be run in a virtualenv
    :type python_callable: function
    :param requirements: A list of requirements as specified in a pip install command
    :type requirements: list[str]
    :param python_version: The Python version to run the virtualenv with. Note that
        both 2 and 2.7 are acceptable forms.
    :type python_version: Optional[Union[str, int, float]]
    :param use_dill: Whether to use dill to serialize
        the args and result (pickle is default). This allow more complex types
        but requires you to include dill in your requirements.
    :type use_dill: bool
    :param system_site_packages: Whether to include
        system_site_packages in your virtualenv.
        See virtualenv documentation for more information.
    :type system_site_packages: bool
    :param op_args: A list of positional arguments to pass to python_callable.
    :type op_args: list