Update development guide to use correct python image

[krivard]

The current guide refers to the python image in operations, which is no longer used in this repository.

The following quick directions I stole from the frontend guide point to the correct docker file to use for python images in delphi-epidata development:

The below was written before we switched the delphi-epidata API server to Python/Flask. If you are following this guide in order to develop on delphi-epidata, you should build this Python image as delphi_web_python and use it in place of delphi_python, and use this web image for delphi_epidata_web. See the CI recipe for the latest instructions on how to build, test, and run locally.