**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
devel_hadoop = devel_minreq + hdfs + hive + kerberos + presto + webhdfs
PROVIDERS_REQUIREMENTS: Dict[str, List[str]] = {
    'airbyte': [],
    'amazon': amazon,
    'apache.beam': apache_beam,
    'apache.cassandra': cassandra,
    'apache.druid': druid,
    'apache.hdfs': hdfs,
    'apache.hive': hive,
    'apache.kylin': kylin,
    'apache.livy': [],
    'apache.pig': [],
    'apache.pinot': pinot,
    'apache.spark': spark,
    'apache.sqoop': [],
    'celery': celery,
    'cloudant': cloudant,
    'cncf.kubernetes': kubernetes,
    'databricks': databricks,
    'datadog': datadog,
    'dingding': [],
    'discord': [],
    'docker': docker,
    'elasticsearch': elasticsearch,
    'exasol': exasol,
    'facebook': facebook,
    'ftp': [],
    'google': google,
    'grpc': grpc,
    'hashicorp': hashicorp,