Not Flask, but still Python: Wagtail - https://github.com/wagtail/wagtail - is a popular headless CMS option. Michael Harrison from rice.edu / Openstax gave a great talk about this at last week's Wagtail Space:
big thanks. Actually I worked for Openstax (Connexions) in the past but I think I did not met Michael Harrison at my time :)) I will definitely give it a testrun!