**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
"""
    List all available extras
    Registered as cmdclass in setup() so it can be called with ``python setup.py list_extras``.
    """
    description = "List available extras"
    user_options: List[str] = []
    def initialize_options(self):
        """Set default values for options."""
    def finalize_options(self):
        """Set final values for options."""
    def run(self):  
        """List extras."""
        print("\n".join(wrap(", ".join(EXTRAS_REQUIREMENTS.keys()), 100)))
def git_version(version_: str) -> str:
    """
    Return a version to identify the state of the underlying git repo. The version will
    indicate whether the head of the current git-backed working directory is tied to a
    release tag or not : it will indicate the former with a 'release:{version}' prefix
    and the latter with a 'dev0' prefix. Following the prefix will be a sha of the current
    branch head. Finally, a "dirty" suffix is appended to indicate that uncommitted
    changes are present.
    :param str version_: Semver version
    :return: Found Airflow version in Git repo
    :rtype: str