**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
When installing providers from sources we install all namespace packages found below airflow,
        including airflow and provider packages, otherwise defaults from setup.cfg control this.
        The kwargs in setup() call override those that are specified in setup.cfg.
        """
        if os.getenv(INSTALL_PROVIDERS_FROM_SOURCES) == 'true':
            setup_kwargs['packages'] = find_namespace_packages(include=['airflow*'])
    include_provider_namespace_packages_when_installing_from_sources()
    if os.getenv(INSTALL_PROVIDERS_FROM_SOURCES) == 'true':
        print("Installing providers from sources. Skip adding providers as dependencies")
    else:
        add_all_provider_packages()
    write_version()
    setup(
        distclass=AirflowDistribution,
        version=version,
        extras_require=EXTRAS_REQUIREMENTS,
        download_url=('https://archive.apache.org/dist/airflow/' + version),
        cmdclass={
            'extra_clean': CleanCommand,
            'compile_assets': CompileAssets,
            'list_extras': ListExtras,
            'install': Install,
            'develop': Develop,
        },
        test_suite='setup.airflow_test_suite',
        **setup_kwargs,
    )
if __name__ == "__main__":
    do_setup()