**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
'http': [],
    'imap': [],
    'jdbc': jdbc,
    'jenkins': jenkins,
    'jira': jira,
    'microsoft.azure': azure,
    'microsoft.mssql': mssql,
    'microsoft.winrm': winrm,
    'mongo': mongo,
    'mysql': mysql,
    'neo4j': neo4j,
    'odbc': odbc,
    'openfaas': [],
    'opsgenie': [],
    'oracle': oracle,
    'pagerduty': pagerduty,
    'papermill': papermill,
    'plexus': plexus,
    'postgres': postgres,
    'presto': presto,
    'qubole': qubole,
    'redis': redis,
    'salesforce': salesforce,
    'samba': samba,
    'segment': segment,
    'sendgrid': sendgrid,
    'sftp': ssh,
    'singularity': singularity,
    'slack': slack,
    'snowflake': snowflake,