First, set up a web app using any framework and Python version (it doesn't matter which, since we won't need them. Maybe just choose Manual Configuration and Python3.5).
Then, on the Web tab configuration screen, scroll down to the Static Files section, and add one new entry:
- URL: /
- Path: /home/yourusername/path/to/your/static/site
Hit Reload on the web tab.
Then, any files in /home/yourusername/path/to/your/static/site will be served automatically. Start with /home/yourusername/path/to/your/static/site/index.html for example!