**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
"""
    Return sphinx theme version. If USE_THEME_FROM_GIT env variable is set, the theme is used from
    GitHub to allow dynamically update it during development. However for regular PIP release
    you cannot use @ package specification, so the latest available released theme package from
    PIP is used.
    :return: Version of sphinx theme to use.
    """
    if os.environ.get('USE_THEME_FROM_GIT'):
        return (
            "@ https://github.com/apache/airflow-site/releases/download/0.0.4/"
            + "sphinx_airflow_theme-0.0.4-py3-none-any.whl"
        )
    return ''
amazon = [
    'boto3>=1.15.0,<1.18.0',
    'watchtower~=0.7.3',
]
apache_beam = [
    'apache-beam[gcp]',
]
async_packages = [
    'eventlet>= 0.9.7',
    'gevent>=0.13',
    'greenlet>=0.4.9',
]
atlas = [
    'atlasclient>=0.1.2',
]