**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
'hmsclient>=0.1.0',
    'pyhive[hive]>=0.6.0',
    'thrift>=0.9.2',
]
jdbc = [
    'jaydebeapi>=1.1.1',
]
jenkins = [
    'python-jenkins>=1.0.0',
]
jira = [
    'JIRA>1.0.7',
]
kerberos = [
    'pykerberos>=1.1.13',
    'requests_kerberos>=0.10.0',
    'thrift_sasl>=0.2.0',
]
kubernetes = [
    'cryptography>=2.0.0',
    'kubernetes>=3.0.0, <12.0.0',
]
kylin = ['kylinpy>=2.6']
ldap = [
    'ldap3>=2.5.1',
    'python-ldap',
]
mongo = [
    'dnspython>=1.13.0,<2.0.0',
    'pymongo>=3.6.0',