Hello,

Slack has released updates to its library and we can start using it for it. 

We especially like one change.

> slack_sdk has no required dependencies. This means aiohttp is no longer automatically resolved.

I've looked through the documentation and it doesn't look like a difficult task, but I think it's still worth testing.
More info: https://slack.dev/python-slack-sdk/v3-migration/index.html#from-slackclient-2-x

Best regards,
Kamil Breguła
setup.py
"""
    Return sphinx theme version. If USE_THEME_FROM_GIT env variable is set, the theme is used from
    GitHub to allow dynamically update it during development. However for regular PIP release
    you cannot use @ package specification, so the latest available released theme package from
    PIP is used.
    :return: Version of sphinx theme to use.
    """
    if os.environ.get('USE_THEME_FROM_GIT'):
        return (
            "@ https://github.com/apache/airflow-site/releases/download/0.0.4/"
            + "sphinx_airflow_theme-0.0.4-py3-none-any.whl"
        )
    return ''
amazon = [
    'boto3>=1.15.0,<1.16.0',
    'botocore>=1.18.0,<1.19.0',
    'watchtower~=0.7.3',
]
apache_beam = [
    'apache-beam[gcp]',
]
async_packages = [
    'eventlet>= 0.9.7',
    'gevent>=0.13',
    'greenlet>=0.4.9',
]