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=["sympy"])
def test_sympy(selenium):
    import sympy

    a, b = sympy.symbols("a,b")
    c = sympy.sqrt(a**2 + b**2)

    assert c.subs({a: 3, b: 4}) == 5

from pytest_pyodide import run_in_pyodide


@run_in_pyodide(packages=["sympy"])
def test_sympy(selenium):
    import sympy

    a, b = sympy.symbols("a,b")
    c = sympy.sqrt(a**2 + b**2)

    assert c.subs({a: 3, b: 4}) == 5
