**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
]
mssql = [
    'pymssql~=2.1,>=2.1.5',
]
mysql = [
    'mysql-connector-python>=8.0.11, <=8.0.22',
    'mysqlclient>=1.3.6,<3',
]
neo4j = ['neo4j>=4.2.1']
odbc = [
    'pyodbc',
]
oracle = [
    'cx_Oracle>=5.1.2',
]
pagerduty = [
    'pdpyras>=4.1.2,<5',
]
papermill = [
    'nteract-scrapbook[all]>=0.3.1',
    'papermill[all]>=1.2.1',
]
password = [
    'bcrypt>=2.0.0',
    'flask-bcrypt>=0.7.1',
]
pinot = [
    
    
    'pinotdb>0.1.2,<1.0.0',