**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
```

airflow/serialization/serialized_objects.py
type_ = encoded_var[Encoding.TYPE]
        if type_ == DAT.DICT:
            return {k: cls._deserialize(v) for k, v in var.items()}
        elif type_ == DAT.DAG:
            return SerializedDAG.deserialize_dag(var)
        elif type_ == DAT.OP:
            return SerializedBaseOperator.deserialize_operator(var)
        elif type_ == DAT.DATETIME:
            return pendulum.from_timestamp(var)
        elif type_ == DAT.POD:
            pod = PodGenerator.deserialize_model_dict(var)
            return pod
        elif type_ == DAT.TIMEDELTA:
            return datetime.timedelta(seconds=var)
        elif type_ == DAT.TIMEZONE:
            return Timezone(var)
        elif type_ == DAT.RELATIVEDELTA:
            if 'weekday' in var:
                var['weekday'] = relativedelta.weekday(*var['weekday'])  
            return relativedelta.relativedelta(**var)
        elif type_ == DAT.SET:
            return {cls._deserialize(v) for v in var}
        elif type_ == DAT.TUPLE:
            return tuple([cls._deserialize(v) for v in var])
        else:
            raise TypeError('Invalid type {!s} in deserialization.'.format(type_))
    _deserialize_datetime = pendulum.from_timestamp
    _deserialize_timezone = pendulum.tz.timezone
    @classmethod
    def _deserialize_timedelta(cls, seconds: int) -> datetime.timedelta: