**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
'github3.py',
    'gitpython',
    'importlib-resources~=1.4',
    'ipdb',
    'jira',
    'jsonpath-ng',
    'jsondiff',
    'mongomock',
    'moto~=2.0',
    'mypy==0.770',
    'parameterized',
    'paramiko',
    'pipdeptree',
    'pre-commit',
    'pylint>=2.7.0',
    'pysftp',
    'pytest~=6.0',
    'pytest-cov',
    'pytest-instafail',
    'pytest-rerunfailures~=9.1',
    'pytest-timeouts',
    'pytest-xdist',
    'python-jose',
    'pywinrm',
    'qds-sdk>=1.9.6',
    'requests_mock',
    'wheel',
    'yamllint',
]
devel_minreq = cgroups + devel + doc + kubernetes + mysql + password