Fix emscripten CI for Chrome 137+ - #3599

@illia-v I think this is ready to go now I got rid of the silly test code in fetch.py

@joemarshall thanks for the efforts to fix the CI!
I've reviewed the changes and understand that code of urllib3 (other than tests) is not affected by JSPI becoming default in Chrome. I found some places getting confusing now when an environment can support both JSPI and XHR by default, and I need to spend more time on this next week. Let's use Chrome stable in the meantime

On second thought, I'm ready to merge this with a couple of minor changes. There is still some refactoring I'd like to do on top of this, but it can be done separately

I don't understand why we need pyodide.ffi.can_run_sync = lambda: False twice, but it does fix CI.

I believe one patches tests using web workers and another for tests running in the main thread