**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
databricks = [
    'requests>=2.20.0, <3',
]
datadog = [
    'datadog>=0.14.0',
]
doc = [
    
    'sphinx>=2.1.2, <3.5.0',
    f'sphinx-airflow-theme{get_sphinx_theme_version()}',
    'sphinx-argparse>=0.1.13',
    'sphinx-autoapi==1.0.0',
    'sphinx-copybutton',
    'sphinx-jinja~=1.1',
    'sphinx-rtd-theme>=0.1.6',
    'sphinxcontrib-httpdomain>=1.7.0',
    'sphinxcontrib-redoc>=1.6.0',
    'sphinxcontrib-spelling==5.2.1',
]
docker = [
    'docker~=3.0',
]
druid = [
    'pydruid>=0.4.1',
]
elasticsearch = [
    'elasticsearch>7, <7.6.0',
    'elasticsearch-dbapi==0.1.0',
    'elasticsearch-dsl>=5.0.0',
]