**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
'vertica-python>=0.5.1',
]
virtualenv = [
    'virtualenv',
]
webhdfs = [
    'hdfs[avro,dataframe,kerberos]>=2.0.4',
]
winrm = [
    'pywinrm~=0.4',
]
yandex = [
    'yandexcloud>=0.22.0',
]
zendesk = [
    'zdesk',
]
devel = [
    'aws_xray_sdk',
    'beautifulsoup4~=4.7.1',
    'black',
    'blinker',
    'bowler',
    'click~=7.1',
    'coverage',
    'docutils',
    'flake8>=3.6.0',
    'flake8-colors',
    'flaky',
    'freezegun',