**What happened**:

`pandas-gbq` released version [0.15.0](https://github.com/pydata/pandas-gbq/releases/tag/0.15.0) which broke `apache-airflow-backport-providers-google==2020.11.23`

```
../lib/python3.7/site-packages/airflow/providers/google/cloud/hooks/bigquery.py:49: in <module>
    from pandas_gbq.gbq import (
E   ImportError: cannot import name '_check_google_client_version' from 'pandas_gbq.gbq' (/usr/local/lib/python3.7/site-packages/pandas_gbq/gbq.py)
```

The fix is to pin `pandas-gpq==0.14.1`.
setup.py
'sqlite': [],
    'ssh': ssh,
    'tableau': tableau,
    'telegram': telegram,
    'vertica': vertica,
    'yandex': yandex,
    'zendesk': zendesk,
}
ADDITIONAL_EXTRAS_REQUIREMENTS: Dict[str, List[str]] = {
    'apache.atlas': atlas,
    'apache.webhdfs': webhdfs,
}
CORE_EXTRAS_REQUIREMENTS: Dict[str, List[str]] = {
    'async': async_packages,
    'celery': celery,  
    'cgroups': cgroups,
    'cncf.kubernetes': kubernetes,  
    'dask': dask,
    'github_enterprise': flask_oauth,
    'google_auth': flask_oauth,
    'kerberos': kerberos,
    'ldap': ldap,
    'password': password,
    'rabbitmq': rabbitmq,
    'sentry': sentry,
    'statsd': statsd,
    'virtualenv': virtualenv,
}
EXTRAS_REQUIREMENTS: Dict[str, List[str]] = deepcopy(CORE_EXTRAS_REQUIREMENTS)
def add_extras_for_all_providers() -> None: