Upgrade to Flask 3

[jotaen4tinypilot]

Flask 3 was released in September 2023. We are still on Flask 2 right now.

Flask is our web server library, so it’s probably one of our most important dependencies. Also, a lot of the indirect Python dependencies are brought in via Flask, e.g. Jinja or Werkzeug. So by upgrading Flask, we can potentially upgrade a lot of our other dependencies as well.

I already naively tried the upgrade, but there seem to be breaking API changes in the library which affect our code, which we have to look into more closely. The server process crashed upon startup, because due to methods having been renamed/removed in the flask_wtf package that we apparently depend on. I didn’t investigate further.