**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
azure = [
    'azure-batch>=8.0.0',
    'azure-cosmos>=3.0.1,<4',
    'azure-datalake-store>=0.0.45',
    'azure-identity>=1.3.1',
    'azure-keyvault>=4.1.0',
    'azure-kusto-data>=0.0.43,<0.1',
    'azure-mgmt-containerinstance>=1.5.0,<2.0',
    'azure-mgmt-datafactory>=1.0.0,<2.0',
    'azure-mgmt-datalake-store>=0.5.0',
    'azure-mgmt-resource>=2.2.0',
    'azure-storage-blob>=12.7.0',
    'azure-storage-common>=2.1.0',
    'azure-storage-file>=2.1.0',
]
cassandra = [
    'cassandra-driver>=3.13.0,<4',
]
celery = [
    'celery~=4.4.2',
    'flower>=0.7.3, <1.0',
    'vine~=1.3',  
]
cgroups = [
    'cgroupspy>=0.1.4',
]
cloudant = [
    'cloudant>=2.0',
]
dask = ['cloudpickle>=1.4.1, <1.5.0', 'distributed>=2.11.1, <2.20']