**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
"""
    Command to tidy up the project root.
    Registered as cmdclass in setup() so it can be called with ``python setup.py extra_clean``.
    """
    description = "Tidy up the project root"
    user_options: List[str] = []
    def initialize_options(self):
        """Set default values for options."""
    def finalize_options(self):
        """Set final values for options."""
    @staticmethod
    def rm_all_files(files: List[str]):
        """Remove all files from the list"""
        for file in files:
            try:
                os.remove(file)
            except Exception as e:  
                logger.warning("Error when removing %s: %s", file, e)
    def run(self):
        """Remove temporary files and directories."""
        os.chdir(my_dir)
        self.rm_all_files(glob.glob('./build/*'))
        self.rm_all_files(glob.glob('./**/__pycache__/*', recursive=True))
        self.rm_all_files(glob.glob('./**/*.pyc', recursive=True))
        self.rm_all_files(glob.glob('./dist/*'))
        self.rm_all_files(glob.glob('./*.egg-info'))
        self.rm_all_files(glob.glob('./docker-context-files/*.whl'))
        self.rm_all_files(glob.glob('./docker-context-files/*.tgz'))
class CompileAssets(Command):