**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
]
plexus = [
    'arrow>=0.16.0,<1.0.0',
]
postgres = [
    'psycopg2-binary>=2.7.4',
]
presto = ['presto-python-client>=0.7.0,<0.8']
qubole = [
    'qds-sdk>=1.10.4',
]
rabbitmq = [
    'amqp<5.0.0',
]
redis = [
    'redis~=3.2',
]
salesforce = [
    'simple-salesforce>=1.0.0',
    'tableauserverclient',
]
samba = [
    'pysmbclient>=0.1.3',
]
segment = [
    'analytics-python>=1.2.9',
]
sendgrid = [
    'sendgrid>=6.0.0,<7',
]