Upload from Photos to Flickr does not work.
Photo's version 2.0 on OS 10.12.5.
The change from "iPhotos" to "Photos", the "direct to Flickr" sharing was broken, another feature lost from iPhoto.
I've seen some people say that all you need to do is add Flickr under Internet Accounts, and I can confirm that under 10.12.5, there is still a Flickr account, which I added. But that doesn't change anything under Photos (2.0). Selecting an album and using File -> Share, on the popup, I still only see: "iCloud Photo Sharing", "Mail", "AirDrop", "Notes", and "More...". Selecting "More..." pulls up an "Extensions" menu, with a checkbox to "Use Apple and third-party extensions to customize your Mac." Under "Select extensions for sharing with others" I see "Flickr" (after I added it to Internet Accounts), and it's CHECKED. But there is nothing in this pop-up that lets you actually upload/sync the album to Flickr like you used to be able to do. (Note that the little "share" up-arrow in the upper-right corner of "Photos" is functionally equivalent to "File -> Share".