**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
'google-cloud-os-login>=2.0.0,<3.0.0',
    'google-cloud-pubsub>=2.0.0,<3.0.0',
    'google-cloud-redis>=2.0.0,<3.0.0',
    'google-cloud-secret-manager>=0.2.0,<2.0.0',
    'google-cloud-spanner>=1.10.0,<2.0.0',
    'google-cloud-speech>=0.36.3,<2.0.0',
    'google-cloud-storage>=1.30,<2.0.0',
    'google-cloud-tasks>=2.0.0,<3.0.0',
    'google-cloud-texttospeech>=0.4.0,<2.0.0',
    'google-cloud-translate>=1.5.0,<2.0.0',
    'google-cloud-videointelligence>=1.7.0,<2.0.0',
    'google-cloud-vision>=0.35.2,<2.0.0',
    'google-cloud-workflows>=0.1.0,<2.0.0',
    'grpcio-gcp>=0.2.2',
    'json-merge-patch~=0.2',
    'pandas-gbq',
    'plyvel',
]
grpc = [
    'google-auth>=1.0.0, <2.0.0dev',
    'google-auth-httplib2>=0.0.1',
    'grpcio>=1.15.0',
]
hashicorp = [
    'hvac~=0.10',
]
hdfs = [
    'snakebite-py3',
]
hive = [