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
docs/conf.py
PACKAGE_VERSION = 'master'
os.environ['AIRFLOW_PACKAGE_NAME'] = PACKAGE_NAME
if PACKAGE_DIR:
    os.environ['AIRFLOW_PACKAGE_DIR'] = PACKAGE_DIR
os.environ['AIRFLOW_PACKAGE_VERSION'] = PACKAGE_VERSION
os.environ['BUILDING_AIRFLOW_DOCS'] = 'TRUE'
project = PACKAGE_NAME
version = PACKAGE_VERSION
release = PACKAGE_VERSION
extensions = [
    'provider_init_hack',
    'sphinx.ext.autodoc',
    'sphinx.ext.viewcode',
    'sphinxarg.ext',
    'sphinx.ext.intersphinx',
    'exampleinclude',
    'docroles',
    'removemarktransform',
    'sphinx_copybutton',
    'airflow_intersphinx',
    "sphinxcontrib.spelling",
    'sphinx_airflow_theme',
    'redirects',
]
if PACKAGE_NAME == 'apache-airflow':
    extensions.extend(
        [
            'sphinxcontrib.jinja',
            'sphinx.ext.graphviz',
            'sphinxcontrib.httpdomain',