**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
#
#
#
"""Setup.py for the Airflow project."""
import glob
import logging
import os
import subprocess
import unittest
from copy import deepcopy
from distutils import log
from os.path import dirname, relpath
from textwrap import wrap
from typing import Dict, List, Tuple
from setuptools import Command, Distribution, find_namespace_packages, setup
from setuptools.command.develop import develop as develop_orig
from setuptools.command.install import install as install_orig
INSTALL_PROVIDERS_FROM_SOURCES = 'INSTALL_PROVIDERS_FROM_SOURCES'
logger = logging.getLogger(__name__)
version = '2.1.0.dev0'
my_dir = dirname(__file__)
def airflow_test_suite() -> unittest.TestSuite:
    """Test suite for Airflow tests"""
    test_loader = unittest.TestLoader()
    test_suite = test_loader.discover(os.path.join(my_dir, 'tests'), pattern='test_*.py')
    return test_suite
class CleanCommand(Command):