Downloading all photos from iCloud Photo Library without Photos app
I highly recommend ndbroadbent's fantastic Python program the icloud_photos_downloader project. The project repository can be found at his Github account: https://github.com/ndbroadbent/icloud_photos_downloader. 

There are some great instructions on installation and use in the project README. Once installed, you run the program to download all your photos to a directory called foto_folder; from the terminal 

./download_photos.py ../foto_folder 
    --username=myemail@email.com
    --password=myp@ss
    --size=original
    --download-videos

And viola, it does its magic.

I've also posted this answer here: https://apple.stackexchange.com/a/311787/271502