Use loky.cpu_count() instead of os.cpu_count(). <!-- Thank you for contributing to Pyodide! All improvements are welcome,
     so don't be afraid to make a PR. -->

### Description

As discussed in https://github.com/pyodide/pyodide/pull/3559#issuecomment-1435965170

### Checklists

- [x] Add a [CHANGELOG](https://github.com/pyodide/pyodide/blob/main/docs/project/changelog.md) entry
- [x] Add / update tests

from pytest_pyodide import run_in_pyodide


@run_in_pyodide(packages=["msgpack"])
def test_pack(selenium_standalone):
    from msgpack import Packer, Unpacker, pack, packb, unpackb  # noqa: F401

    def check(data, use_list=False):
        re = unpackb(packb(data), use_list=use_list, strict_map_key=False)
        assert re == data

    test_data = [
        0,
        1,
        127,
        128,
        255,
        256,
        65535,
        65536,
        4294967295,
        4294967296,
        -1,
        -32,
        -33,
        -128,
        -129,
        -32768,
        -32769,
        -4294967296,
        -4294967297,
        1.0,
        b"",
        b"a",
        b"a" * 31,
        b"a" * 32,
        None,
        True,
        False,
        (),
        ((),),
        ((), None),
        {None: 0},
        (1 << 23),
    ]
    for td in test_data:
        check(td)

from pytest_pyodide import run_in_pyodide


@run_in_pyodide(packages=["msgpack"])
def test_pack(selenium_standalone):
    from msgpack import Packer, Unpacker, pack, packb, unpackb  # noqa: F401

    def check(data, use_list=False):
        re = unpackb(packb(data), use_list=use_list, strict_map_key=False)
        assert re == data

    test_data = [
        0,
        1,
        127,
        128,
        255,
        256,
        65535,
        65536,
        4294967295,
        4294967296,
        -1,
        -32,
        -33,
        -128,
        -129,
        -32768,
        -32769,
        -4294967296,
        -4294967297,
        1.0,
        b"",
        b"a",
        b"a" * 31,
        b"a" * 32,
        None,
        True,
        False,
        (),
        ((),),
        ((), None),
        {None: 0},
        (1 << 23),
    ]
    for td in test_data:
        check(td)
