I assume you don’t mean to open the bundle directory directly and start working in the inner data structures of Apple Photos?
That’s actually one way to do it, even though it’s neither practical, nor really ‘clean’. 
The best way I could recommend would be to use “Edit With” in Photos to open the photo in PhotoLab, but even then, it will still generate a picture in a temporary folder within the Photos library bundle, so we still need to get inside there.
When that happens, either way, it should prompt you the first time for permission to access Photos. If you don’t get that prompt, you’ll have to go into System Preferences > Security > Privacy > Photos, and there you can enable PhotoLab to allow the access.