**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
"""
    Adds extras for all providers.
    By default all providers have the same extra name as provider id, for example
    'apache.hive' extra has 'apache.hive' provider requirement.
    """
    for provider_name, provider_requirement in PROVIDERS_REQUIREMENTS.items():
        EXTRAS_REQUIREMENTS[provider_name] = provider_requirement
def add_additional_extras() -> None: