Expand Python backend to a full package

[penelopeysm]

Followup from #16.

Things to include:

• [ ] Time estimate of what's necessary to achieve the following
• [x] Expand the Python code from a single file into a folder that can be packaged and e.g. uploaded to PyPI. We don't necessarily want to actually upload the template package to PyPI, but someone using the template might want to.
• [x] Set up infrastructure to build the wheel so that it can then be imported with micropip.
  • [ ] Not done: See next comment
• [ ] Document how to ensure compatibility with pyodide's dependencies https://pyodide.org/en/stable/usage/packages-in-pyodide.html

If it's helpful, our previous work on DemoLand:

• The Python package is its own repo, https://github.com/Urban-Analytics-Technology-Platform/demoland-engine
• The wheel can be built on GitHub Actions (it can in principle be done locally too; I just found it easier to build the wheel online because each version of Pyodide requires a very specific Python version and I didn't want to have to install it locally) https://github.com/Urban-Analytics-Technology-Platform/demoland-engine/blob/main/.github/workflows/build_wasm_wheel.yaml
• The TypeScript code that imports the Python package is here: https://github.com/Urban-Analytics-Technology-Platform/demoland-web/tree/main/src/lib/python

[penelopeysm]

27 generated a very simple Python package, see https://github.com/Urban-Analytics-Technology-Platform/web-app-template/tree/main/packages/create-web/template/python_backend

However the problem remains that the name of the wheel file is hardcoded in the TypeScript code. A simple thing like changing the name of the package or the version number will cause the code to break

https://github.com/Urban-Analytics-Technology-Platform/web-app-template/blob/48225ef5ccfc67002c37dbbb69f1244016a37dd6/packages/create-web/template/web/src/lib/python_worker.ts#L29-L33

It's not possible to query the filesystem from a frontend (you need a real backend server — not our fake WASM backends to do that) so we can't do the equivalent of ls *.whl and take the output of that.

My current best idea is:

• when building the wheel with pip (i.e. at the build stage) we somehow get the name of the wheel that's generated. I don't know if pip can do this or if we need to do some hacky thing like parsing pyproject.toml.
• dump the name of the wheel into a json file somewhere inside the static dir at a known file path, it could be something like static/wheel.json and its contents would just be a single string
• the frontend can then import that json file and look inside to get the path to the wheel, then micropip install that