**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
exasol = [
    'pyexasol>=0.5.1,<1.0.0',
]
facebook = [
    'facebook-business>=6.0.2',
]
flask_oauth = [
    'Flask-OAuthlib>=0.9.1,<0.9.6',  
    'oauthlib!=2.0.3,!=2.0.4,!=2.0.5,<3.0.0,>=1.1.2',
    'requests-oauthlib<1.2.0',
]
google = [
    'PyOpenSSL',
    'google-ads>=4.0.0,<8.0.0',
    'google-api-core>=1.25.1,<2.0.0',
    'google-api-python-client>=1.6.0,<2.0.0',
    'google-auth>=1.0.0,<2.0.0',
    'google-auth-httplib2>=0.0.1',
    'google-cloud-automl>=2.1.0,<3.0.0',
    'google-cloud-bigquery-datatransfer>=3.0.0,<4.0.0',
    'google-cloud-bigtable>=1.0.0,<2.0.0',
    'google-cloud-container>=0.1.1,<2.0.0',
    'google-cloud-datacatalog>=3.0.0,<4.0.0',
    'google-cloud-dataproc>=2.2.0,<3.0.0',
    'google-cloud-dlp>=0.11.0,<2.0.0',
    'google-cloud-kms>=2.0.0,<3.0.0',
    'google-cloud-language>=1.1.1,<2.0.0',
    'google-cloud-logging>=2.1.1,<3.0.0',
    'google-cloud-memcache>=0.2.0',
    'google-cloud-monitoring>=2.0.0,<3.0.0',