**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
"""
    try:
        import git
        try:
            repo = git.Repo(os.path.join(*[my_dir, '.git']))
        except git.NoSuchPathError:
            logger.warning('.git directory not found: Cannot compute the git version')
            return ''
        except git.InvalidGitRepositoryError:
            logger.warning('Invalid .git directory not found: Cannot compute the git version')
            return ''
    except ImportError:
        logger.warning('gitpython not found: Cannot compute the git version.')
        return ''
    if repo:
        sha = repo.head.commit.hexsha
        if repo.is_dirty():
            return f'.dev0+{sha}.dirty'
        
        return f'.release:{version_}+{sha}'
    return 'no_git_version'
def write_version(filename: str = os.path.join(*[my_dir, "airflow", "git_version"])):
    """
    Write the Semver version + git hash to file, e.g. ".dev0+2f635dc265e78db6708f59f68e8009abb92c1e65".
    :param str filename: Destination file to write
    """
    text = f"{git_version(version)}"
    with open(filename, 'w') as file:
        file.write(text)
def get_sphinx_theme_version() -> str: