Accessing Yahoo Mail through Gmail with two factor authentication

Yahoo mail was previously set up in my Gmail account using the Accounts and Import part of Settings:

However, enabling two factor authentication on Yahoo (Settings > Account Security) stops this working.

The solution is - still within Yahoo Account Security - to add an App Specific Password 

Go to Manage app passwords:

I have added an option of Gmail. This produces a long password which is then used in Gmail *instead* of the Yahoo password - this acts as two factor authentication.