**Apache Airflow version**: 2.0.0b2

**What happened**:

Starting in 2.0.0b2, extras that aren't also providers like `statsd` prevents Airflow from being installed.

**How to reproduce it**:
```
$ docker run --rm -ti python:3.6 bash
# pip install apache-airflow[statsd]==2.0.0b2
Collecting apache-airflow[statsd]==2.0.0b2                 
  Downloading apache_airflow-2.0.0b2-py3-none-any.whl (4.5 MB)                    
...
Collecting statsd<4.0,>=3.3.0; extra == "statsd"          
  Downloading statsd-3.3.0-py2.py3-none-any.whl (11 kB)
ERROR: Could not find a version that satisfies the requirement apache-airflow-providers-statsd; extra == "statsd" (from apache-airflow[statsd]==2.0.0b2) (from versions: none)         
ERROR: No matching distribution found for apache-airflow-providers-statsd; extra == "statsd" (from apache-airflow[statsd]==2.0.0b2)
```

I believe this is from https://github.com/apache/airflow/pull/12233
setup.py
'Environment :: Console',
            'Environment :: Web Environment',
            'Intended Audience :: Developers',
            'Intended Audience :: System Administrators',
            'License :: OSI Approved :: Apache Software License',
            'Programming Language :: Python :: 3.6',
            'Programming Language :: Python :: 3.7',
            'Programming Language :: Python :: 3.8',
            'Topic :: System :: Monitoring',
        ],
        author='Apache Software Foundation',
        author_email='dev@airflow.apache.org',
        url='http://airflow.apache.org/',
        download_url=('https://archive.apache.org/dist/airflow/' + version),
        cmdclass={
            'extra_clean': CleanCommand,
            'compile_assets': CompileAssets,
            'list_extras': ListExtras,
        },
        test_suite='setup.airflow_test_suite',
        python_requires='~=3.6',
        project_urls={
            'Documentation': 'https://airflow.apache.org/docs/',
            'Bug Tracker': 'https://github.com/apache/airflow/issues',
            'Source Code': 'https://github.com/apache/airflow',
        },
    )
if __name__ == "__main__":
    do_setup()