**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
"""
    The setuptools.Distribution subclass with Airflow specific behaviour
    The reason for pylint: disable=signature-differs of parse_config_files is explained here:
    https://github.com/PyCQA/pylint/issues/3737
    """
    def parse_config_files(self, *args, **kwargs):  
        """
        Ensure that when we have been asked to install providers from sources
        that we don't *also* try to install those providers from PyPI.
        Also we should make sure that in this case we copy provider.yaml files so that
        Providers manager can find package information.
        """
        super().parse_config_files(*args, **kwargs)
        if os.getenv(INSTALL_PROVIDERS_FROM_SOURCES) == 'true':
            self.install_requires = [  
                req for req in self.install_requires if not req.startswith('apache-airflow-providers-')
            ]
            provider_yaml_files = glob.glob("airflow/providers/**/provider.yaml", recursive=True)
            for provider_yaml_file in provider_yaml_files:
                provider_relative_path = relpath(provider_yaml_file, os.path.join(my_dir, "airflow"))
                self.package_data['airflow'].append(provider_relative_path)
        else:
            self.install_requires.extend(
                [get_provider_package_from_package_id(package_id) for package_id in PREINSTALLED_PROVIDERS]
            )
def replace_extra_requirement_with_provider_packages(extra: str, providers: List[str]) -> None: