**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
"""
    Checks if package should be excluded.
    :param package: package name (beginning of it)
    :param exclusion_list: list of excluded packages
    :return: true if package should be excluded
    """
    return any(package.startswith(excluded_package) for excluded_package in exclusion_list)
devel_all = [
    package
    for package in devel_all
    if not is_package_excluded(package=package, exclusion_list=PACKAGES_EXCLUDED_FOR_ALL)
]
devel_ci = [
    package
    for package in devel_all
    if not is_package_excluded(
        package=package, exclusion_list=PACKAGES_EXCLUDED_FOR_CI + PACKAGES_EXCLUDED_FOR_ALL
    )
]
EXTRAS_REQUIREMENTS["doc"] = doc
EXTRAS_REQUIREMENTS["devel"] = devel_minreq  
EXTRAS_REQUIREMENTS["devel_hadoop"] = devel_hadoop  
EXTRAS_REQUIREMENTS["devel_all"] = devel_all
EXTRAS_REQUIREMENTS["devel_ci"] = devel_ci
def sort_extras_requirements() -> Dict[str, List[str]]: