At my company, we do exactly that using Github+Slack integration. Github Codeowners + Github Teams let's you to autoassign teams as reviewers. When the team is assigned then a random person from the team is pinged using round robin. Furthermore, with Github Scheduled Reminders you can get notifications in Slack according to some schedule and/or when some event happens, e.g. your PR was approved, got a comment or a mention, etc.