**Apache Airflow version**: 2.0.0a1
**Kubernetes version (if you are using kubernetes)** (use `kubectl version`): No kubernetes used.

**Environment**:

- **OS** (e.g. from /etc/os-release): MacOS

**What happened**:

After updating my local dev environment configuration for Airflow 2.0.0a1 I wanted to see if the database needed updating:

```
$ airflow db check
Traceback (most recent call last):
  File "/.../bin/airflow", line 8, in <module>
    sys.exit(main())
  File "/.../lib/python3.6/site-packages/airflow/__main__.py", line 40, in main
    args.func(args)
  File "/.../lib/python3.6/site-packages/airflow/cli/cli_parser.py", line 52, in command
    func = import_string(import_path)
  File "/.../lib/python3.6/site-packages/airflow/utils/module_loading.py", line 32, in import_string
    module = import_module(module_path)
  File "/.../lib/python3.6/importlib/__init__.py", line 126, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 994, in _gcd_import
  File "<frozen importlib._bootstrap>", line 971, in _find_and_load
  File "<frozen importlib._bootstrap>", line 955, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 665, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 678, in exec_module
  File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
  File "/.../lib/python3.6/site-packages/airflow/cli/commands/db_command.py", line 24, in <module>
    from airflow.utils import cli as cli_utils, db
  File "/.../lib/python3.6/site-packages/airflow/utils/db.py", line 26, in <module>
    from airflow.jobs.base_job import BaseJob  # noqa: F401 # pylint: disable=unused-import
  File "/.../lib/python3.6/site-packages/airflow/jobs/__init__.py", line 22, in <module>
    import airflow.jobs.scheduler_job  # noqa
  File "/.../lib/python3.6/site-packages/airflow/jobs/scheduler_job.py", line 50, in <module>
    from airflow.models.serialized_dag import SerializedDagModel
  File "/.../lib/python3.6/site-packages/airflow/models/serialized_dag.py", line 35, in <module>
    from airflow.serialization.serialized_objects import SerializedDAG
  File "/.../lib/python3.6/site-packages/airflow/serialization/serialized_objects.py", line 37, in <module>
    from airflow.kubernetes.pod_generator import PodGenerator
  File "/.../lib/python3.6/site-packages/airflow/kubernetes/pod_generator.py", line 35, in <module>
    from kubernetes.client import models as k8s
ModuleNotFoundError: No module named 'kubernetes'
```

**What you expected to happen**:

`kubernetes` is an optional extra dependency, if it is not installed things should not break.

**How to reproduce it**:

This happens in a new setup, the following steps reproduce the issue:

```python
$ virtualenv -p python3.8 testenv
$ source testenv/bin/activate
$ pip install --find-links https://dist.apache.org/repos/dist/dev/airflow/2.0.0a1/ apache-airflow==2.0.0a1
$ airflow db check
```

tests/serialization/test_dag_serialization.py
"""
        Test TaskGroup serialization/deserialization.
        """
        from airflow.operators.dummy_operator import DummyOperator
        from airflow.utils.task_group import TaskGroup
        execution_date = datetime(2020, 1, 1)
        with DAG("test_task_group_serialization", start_date=execution_date) as dag:
            task1 = DummyOperator(task_id="task1")
            with TaskGroup("group234") as group234:
                _ = DummyOperator(task_id="task2")
                with TaskGroup("group34") as group34:
                    _ = DummyOperator(task_id="task3")
                    _ = DummyOperator(task_id="task4")
            task5 = DummyOperator(task_id="task5")
            task1 >> group234
            group34 >> task5
        dag_dict = SerializedDAG.to_dict(dag)
        SerializedDAG.validate_schema(dag_dict)
        json_dag = SerializedDAG.from_json(SerializedDAG.to_json(dag))
        self.validate_deserialized_dag(json_dag, dag)
        serialized_dag = SerializedDAG.deserialize_dag(SerializedDAG.serialize_dag(dag))
        assert serialized_dag.task_group.children
        assert serialized_dag.task_group.children.keys() == dag.task_group.children.keys()
        def check_task_group(node):