**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
sentry = [
    'blinker>=1.1',
    'sentry-sdk>=0.8.0',
]
singularity = ['spython>=0.0.56']
slack = [
    'slack_sdk>=3.0.0,<4.0.0',
]
snowflake = [
    'snowflake-connector-python>=2.4.1',
    'snowflake-sqlalchemy>=1.1.0',
]
spark = [
    'pyspark',
]
ssh = [
    'paramiko>=2.6.0',
    'pysftp>=0.2.9',
    'sshtunnel>=0.1.4,<0.2',
]
statsd = [
    'statsd>=3.3.0, <4.0',
]
tableau = [
    'tableauserverclient',
]
telegram = [
    'python-telegram-bot==13.0',
]
vertica = [