slack_lite.py

from threading import Thread
from time import sleep

import webview

def load_slack():
    sleep(1)

    # OSX specific
    webkit = webview.cocoa.BrowserView.instance.webkit
    webkit.setCustomUserAgent_((
        'User-Agent: '
        'Mozilla/5.0 (Macintosh; Intel Mac OS X 10_13_0) '
        'AppleWebKit/537.36 (KHTML, like Gecko) '
        'Chrome/61.0.3163.100 Safari/537.36'
    ))

    webview.load_url('https://slack.com/signin')

t = Thread(target=load_slack)
t.start()

# This blocks
webview.create_window('SlackLite', 'about:blank')