Emscripten wheels generate invalid "unreachable"

[ethanhs]

I think this is probably not a bug in mypyc, more of how we build mypy with mypyc for Emscripten. When I set MYPY_FORCE_COLOR=1 in the Emscripten environment, I get the following error:

Traceback (most recent call last):
  File "mypy/api.py", line 71, in run
    return _run(
  File "mypy/api.py", line 59, in _run
    main_wrapper(stdout, stderr)
  File "mypy/api.py", line 72, in <lambda>
    lambda stdout, stderr: main(args=args, stdout=stdout, stderr=stderr, clean_exit=True)
  File "mypy/main.py", line 70, in main
    formatter = util.FancyFormatter(stdout, stderr, options.hide_error_codes)
  File "mypy/util.py", line 547, in __init__
    elif sys.platform == "emscripten":
RuntimeError: Reached allegedly unreachable code!

Which comes from this line in the color output detection code. The error is that the sys.platform is not emscripten as it should be, so I believe the solution is that we need to override the sys.platform when building Emscripten wheels. I want to check with pyodide folks first to make sure there isn't something else we should be doing however.

[JelleZijlstra]

cc @ethanhs

[ethanhs]

I decided this would be better as a change to mypy's options: python/mypy#14888