ci: add job to test with Emscripten

[ariostas]

This PR resumes the work started in #1026. The package is installed and used with pyodide, just how it was done in #1026. I considered switching to using emscripten-forge and xeus-python since there are some significant advantages for deployment, but currently it is much easier to set up these tests with pyodide.

The main difficulty with the tests are network requests and thread spawning. Network issues don't end up being as bad for deployments since often files are hosted on the same server/website (and there's ways to get around CORS protections), so I'll see if I can get some very simple http test working. As for thread spawning, it's more of an issue since there are multiple functions that try to spawn threads without being explicit that they are doing so. Currently WASM doesn't support multiple threads, so various things don't work.

I hand-picked a few test files that work, but probably the best approach will be to introduce a new pytest marker to indicate which tests should be attempted. And maybe it would be good to decide which subset of the functionality should be available in WASM and write some simple tests for it to make it more explicit.

[ariostas]

I think this is starting to be in good shape, so it would be nice to get some feedback.

There are quite a few things that changed since my initial comment.

• Instead of creating a Pyodide venv and running the normal tests, I switched to using pytest-pyodide, which uses selenium to run the Pyodide on an actual web browser so that it is fully-featured and realistic.
• I made a few small changes in reading functions so that they don't spawn extra threads (which is not supported in Pyodide yet, but should be supported later on).
• I added a tests-wasm folder for tests specific to wasm. I wrote a decorator so that we can also use files from skhep_testdata easily. For now, it only tests reading TTree and RNTuple files locally and over http.

[ariostas]

Thank you Jim, that's a great suggestion.

I introduced uproot._util.wasm which is set to True when the platform is Emscripten or WASI. In general, I had to think a bit whether to name things with WASM, Emscripten, or Pyodide suffixes, but I think now it's reasonable.

• uproot._util.wasm is for any WASM platform (Emscripten/WASI) since I would expect that the tweaks I made work everywhere. I'll test on xeus-python (Emscripten-based) at some point, but I'm not sure how to test things on WASI.
• The extra dependencies test-pyodide are specific to Pyodide testing.
• The tests-wasm directory for now only contains Pyodide tests, but I could imagine adding other platforms later and marking them appropriately.