Revamped pyodide support

[kylebarron]

Need to be able to turn off thread creation in the ThreadPoolExecutor

import sys
from concurrent.futures import Executor, Future

class SynchronousExecutor(Executor):
    """Executes immediately on the calling thread."""
    def submit(self, fn, /, *args, **kwargs):
        future = Future()
        try:
            future.set_result(fn(*args, **kwargs))
        except BaseException as exc:
            future.set_exception(exc)
        return future

def make_executor(max_workers=None):
    if sys.platform == "emscripten":
        return SynchronousExecutor()
    from concurrent.futures import ThreadPoolExecutor
    return ThreadPoolExecutor(max_workers=max_workers)

Dependencies

All dependencies now work in pyemscripten, see kylebarron/arro3#502, kylebarron/arro3#504, geoarrow/geoarrow-rs#1448