**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
"""
    For Python 3.6+ the dictionary order remains when keys() are retrieved.
    Sort both: extras and list of dependencies to make it easier to analyse problems
    external packages will be first, then if providers are added they are added at the end of the lists.
    """
    sorted_requirements = dict(sorted(EXTRAS_REQUIREMENTS.items()))  
    for extra_list in sorted_requirements.values():
        extra_list.sort()
    return sorted_requirements
EXTRAS_REQUIREMENTS = sort_extras_requirements()
PREINSTALLED_PROVIDERS = [
    'ftp',
    'http',
    'imap',
    'sqlite',
]
def get_provider_package_from_package_id(package_id: str):
    """
    Builds the name of provider package out of the package id provided/
    :param package_id: id of the package (like amazon or microsoft.azure)
    :return: full name of package in PyPI
    """
    package_suffix = package_id.replace(".", "-")
    return f"apache-airflow-providers-{package_suffix}"
def get_all_provider_packages():
    """Returns all provider packages configured in setup.py"""
    return " ".join([get_provider_package_from_package_id(package) for package in PROVIDERS_REQUIREMENTS])
class AirflowDistribution(Distribution):