tests: run on pyodide

[henryiii]

Adding WebAssembly testing on Pyodide.

Description

Suggested changelog entry:

* Adding WASM testing.

[rwgk]

Do you already understand the Python 3.12 failures? When I saw them previously I decided to trigger a CI run with master as-is (#3939) and I don't see any errors there.

[henryiii]

I don't understand how this could be adding errors on 3.12. No code changes that I see. 🤷

[rwgk]

I don't understand how this could be adding errors on 3.12. No code changes that I see. 🤷

My best guess: some change in the environment triggers a latent bug in 3.12 (or how we use 3.12).

Could tests/pyproject.toml have something to do with it?

[henryiii]

Importing pytest or sys (probably pytest) inside test_embed causes Python 3.12 to segfault. b9b391c fixed the issues - we don't really need to do it, since we are avoiding these anyway on Pyodide. But why? How?

[henryiii]

And the interesting question: Should we add -fexceptions when building on emscripten in our CMake helpers? Is there a reason someone wouldn't want them (using them, which we do a lot, would cause crashes). @hoodmane?

[rwgk]

Awesome to have test coverage for WASM!

Sorry I missed that this PR was marked as ready for review (last time I looked there were still test failures).

Just a minor question:

Did you consider adding something like this to tests/env.py?

 import pytest

+EMSCRIPTEN = sys.platform.startswith("emscripten")
 LINUX = sys.platform.startswith("linux")
 MACOS = sys.platform.startswith("darwin")
 WIN = sys.platform.startswith("win32") or sys.platform.startswith("cygwin")

[henryiii]

I don't like these very much. It makes a bit of sense for WIN (since it's a bit lengthy and combining two conditions - which would be better written sys.platform.startswith(("win32", "cygwin")), and I'm not sure it's right - often Cygwin acts a bit more like Linux), but the others are not really much clearer than the standard expression.