**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
"""Forces installation of providers from sources in editable mode."""
    def run(self):
        self.announce('Standard installation. Providers are installed from packages', level=log.INFO)
        super().run()
def do_setup() -> None:
    """
    Perform the Airflow package setup.
    Most values come from setup.cfg, only the dynamically calculated ones are passed to setup
    function call. See https://setuptools.readthedocs.io/en/latest/userguide/declarative_config.html
    """
    setup_kwargs = {}
    def include_provider_namespace_packages_when_installing_from_sources() -> None:
        """