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**Apache Airflow version**: 2.0.1


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

**Environment**:

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

**What happened**:

While running `airflow connections list` with a `xcom_backend = xcom.MyXComBackend` in `airflow.cfg` I will get a 
 
    ImportError: cannot import name 'BaseHook' from partially initialized module 'airflow.hooks.base' (most likely due to a circular import) (/Users/rubelagu/tmp/airflow-xcom-backend/venv/lib/python3.8/site-packages/airflow/hooks/base.py)

(see below for complete traceback).


The issue seems to be that `airflow/cli/commands/connection_command.py` imports `BaseHook` directly and `BaseHook -> Connection -> BaseOperator -> TaskInstance -> XCom -> MyXComBackend -> S3Hook -> BaseHook ` (again see complete traceback below)
    
```
airflow connections list
[2021-04-12 10:51:34,020] {configuration.py:459} ERROR - cannot import name 'BaseHook' from partially initialized module 'airflow.hooks.base' (most likely due to a circular import) (/Users/rubelagu/tmp/airflow-xcom-backend/venv/lib/python3.8/site-packages/airflow/hooks/base.py)
Traceback (most recent call last):
  File "/Users/rubelagu/tmp/airflow-xcom-backend/venv/lib/python3.8/site-packages/airflow/configuration.py", line 457, in getimport
    return import_string(full_qualified_path)
  File "/Users/rubelagu/tmp/airflow-xcom-backend/venv/lib/python3.8/site-packages/airflow/utils/module_loading.py", line 32, in import_string
    module = import_module(module_path)
  File "/Users/rubelagu/.pyenv/versions/3.8.8/lib/python3.8/importlib/__init__.py", line 127, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1014, in _gcd_import
  File "<frozen importlib._bootstrap>", line 991, in _find_and_load
  File "<frozen importlib._bootstrap>", line 975, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 671, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 783, in exec_module
  File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
  File "/Users/rubelagu/tmp/airflow-xcom-backend/plugins/xcom.py", line 4, in <module>
    from airflow.providers.amazon.aws.hooks.s3 import S3Hook
  File "/Users/rubelagu/tmp/airflow-xcom-backend/venv/lib/python3.8/site-packages/airflow/providers/amazon/aws/hooks/s3.py", line 37, in <module>
    from airflow.providers.amazon.aws.hooks.base_aws import AwsBaseHook
  File "/Users/rubelagu/tmp/airflow-xcom-backend/venv/lib/python3.8/site-packages/airflow/providers/amazon/aws/hooks/base_aws.py", line 41, in <module>
    from airflow.hooks.base import BaseHook
ImportError: cannot import name 'BaseHook' from partially initialized module 'airflow.hooks.base' (most likely due to a circular import) (/Users/rubelagu/tmp/airflow-xcom-backend/venv/lib/python3.8/site-packages/airflow/hooks/base.py)

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/rubelagu/tmp/airflow-xcom-backend/venv/bin/airflow", line 8, in <module>
    sys.exit(main())
  File "/Users/rubelagu/tmp/airflow-xcom-backend/venv/lib/python3.8/site-packages/airflow/__main__.py", line 40, in main
    args.func(args)
  File "/Users/rubelagu/tmp/airflow-xcom-backend/venv/lib/python3.8/site-packages/airflow/cli/cli_parser.py", line 47, in command
    func = import_string(import_path)
  File "/Users/rubelagu/tmp/airflow-xcom-backend/venv/lib/python3.8/site-packages/airflow/utils/module_loading.py", line 32, in import_string
    module = import_module(module_path)
  File "/Users/rubelagu/.pyenv/versions/3.8.8/lib/python3.8/importlib/__init__.py", line 127, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1014, in _gcd_import
  File "<frozen importlib._bootstrap>", line 991, in _find_and_load
  File "<frozen importlib._bootstrap>", line 975, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 671, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 783, in exec_module
  File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
  File "/Users/rubelagu/tmp/airflow-xcom-backend/venv/lib/python3.8/site-packages/airflow/cli/commands/connection_command.py", line 30, in <module>
    from airflow.hooks.base import BaseHook
  File "/Users/rubelagu/tmp/airflow-xcom-backend/venv/lib/python3.8/site-packages/airflow/hooks/base.py", line 23, in <module>
    from airflow.models.connection import Connection
  File "/Users/rubelagu/tmp/airflow-xcom-backend/venv/lib/python3.8/site-packages/airflow/models/__init__.py", line 20, in <module>
    from airflow.models.baseoperator import BaseOperator, BaseOperatorLink
  File "/Users/rubelagu/tmp/airflow-xcom-backend/venv/lib/python3.8/site-packages/airflow/models/baseoperator.py", line 55, in <module>
    from airflow.models.taskinstance import Context, TaskInstance, clear_task_instances
  File "/Users/rubelagu/tmp/airflow-xcom-backend/venv/lib/python3.8/site-packages/airflow/models/taskinstance.py", line 58, in <module>
    from airflow.models.xcom import XCOM_RETURN_KEY, XCom
  File "/Users/rubelagu/tmp/airflow-xcom-backend/venv/lib/python3.8/site-packages/airflow/models/xcom.py", line 289, in <module>
    XCom = resolve_xcom_backend()
  File "/Users/rubelagu/tmp/airflow-xcom-backend/venv/lib/python3.8/site-packages/airflow/models/xcom.py", line 279, in resolve_xcom_backend
    clazz = conf.getimport("core", "xcom_backend", fallback=f"airflow.models.xcom.{BaseXCom.__name__}")
  File "/Users/rubelagu/tmp/airflow-xcom-backend/venv/lib/python3.8/site-packages/airflow/configuration.py", line 460, in getimport
    raise AirflowConfigException(
airflow.exceptions.AirflowConfigException: The object could not be loaded. Please check "xcom_backend" key in "core" section. Current value: "xcom.MyXComBackend".
```

**What you expected to happen**:

I expected to be able to use S3Hook or GCSHook from a cusom XCom backedn following the example in what I though was the canonical example at https://medium.com/apache-airflow/airflow-2-0-dag-authoring-redesigned-651edc397178 



**How to reproduce it**:

* In `airflow.cfg` set `xcom_backend = xcom.MyXComBackend`
* Create file `plugins/xcom.py` with the following contents
```
from airflow.models.xcom import BaseXCom
from airflow.providers.amazon.aws.hooks.s3 import S3Hook

class MyXComBackend(BaseXCom):
    pass
```


**Anything else we need to know**:

This can be workaround by not importing the hooks on the module level but at the method level, but that is
* really ugly
* not evident for anybody trying to create a custom xcom backend for the first time.


```
from airflow.models.xcom import BaseXCom
class MyXComBackend(BaseXCom):


    @staticmethod
    def serialize_value(value):
        from airflow.providers.amazon.aws.hooks.s3 import S3Hook
        hook = S3Hook()
        pass
    @staticmethod
    def deserialize_value(value):
        from airflow.providers.amazon.aws.hooks.s3 import S3Hook
        hook = S3Hook()
        pass
```


airflow/hooks/base.py
#
#
#
"""Base class for all hooks"""
import logging
import warnings
from typing import Any, Dict, List
from airflow.models.connection import Connection
from airflow.typing_compat import Protocol
from airflow.utils.log.logging_mixin import LoggingMixin
log = logging.getLogger(__name__)
class BaseHook(LoggingMixin):