**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
"""Adds extras for all additional extras."""
    for extra_name, extra_requirement in ADDITIONAL_EXTRAS_REQUIREMENTS.items():
        EXTRAS_REQUIREMENTS[extra_name] = extra_requirement
add_extras_for_all_providers()
add_additional_extras()
EXTRAS_DEPRECATED_ALIASES: Dict[str, str] = {
    'atlas': 'apache.atlas',
    'aws': 'amazon',
    'azure': 'microsoft.azure',
    'cassandra': 'apache.cassandra',
    'crypto': '',  
    'druid': 'apache.druid',
    'gcp': 'google',
    'gcp_api': 'google',
    'hdfs': 'apache.hdfs',
    'hive': 'apache.hive',
    'kubernetes': 'cncf.kubernetes',
    'mssql': 'microsoft.mssql',
    'pinot': 'apache.pinot',
    'qds': 'qubole',
    's3': 'amazon',
    'spark': 'apache.spark',
    'webhdfs': 'apache.webhdfs',
    'winrm': 'microsoft.winrm',
}
def find_requirements_for_alias(alias_to_look_for: Tuple[str, str]) -> List[str]: