**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
"""Finds requirements for an alias"""
    deprecated_extra = alias_to_look_for[0]
    new_extra = alias_to_look_for[1]
    if new_extra == '':  
        return []
    try:
        return EXTRAS_REQUIREMENTS[new_extra]
    except KeyError:  
        raise Exception(f"The extra {new_extra} is missing for alias {deprecated_extra}")
def add_extras_for_all_deprecated_aliases() -> None:
    """
    Add extras for all deprecated aliases. Requirements for those deprecated aliases are the same
    as the extras they are replaced with.
    The requirements are not copies - those are the same lists as for the new extras. This is intended.
    Thanks to that if the original extras are later extended with providers, aliases are extended as well.
    """
    for alias, extra in EXTRAS_DEPRECATED_ALIASES.items():
        requirements = EXTRAS_REQUIREMENTS.get(extra) if extra != '' else []
        if requirements is None:
            raise Exception(f"The extra {extra} is missing for deprecated alias {alias}")
        EXTRAS_REQUIREMENTS[alias] = requirements
add_extras_for_all_deprecated_aliases()
ALL_PROVIDERS = list(PROVIDERS_REQUIREMENTS.keys())
ALL_DB_PROVIDERS = [
    'apache.cassandra',
    'apache.druid',