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

package:
  name: atomicwrites
  version: 1.4.1
  top-level:
    - atomicwrites
source:
  sha256: 81b2c9071a49367a7f770170e5eec8cb66567cfbbc8c73d20ce5ca4a8d71cf11
  url: https://files.pythonhosted.org/packages/87/c6/53da25344e3e3a9c01095a89f16dbcda021c609ddb42dd6d7c0528236fb2/atomicwrites-1.4.1.tar.gz
about:
  home: https://github.com/untitaker/python-atomicwrites
  PyPI: https://pypi.org/project/atomicwrites
  summary: Atomic file writes.
  license: MIT

package:
  name: atomicwrites
  version: 1.4.1
  top-level:
    - atomicwrites
source:
  sha256: 81b2c9071a49367a7f770170e5eec8cb66567cfbbc8c73d20ce5ca4a8d71cf11
  url: https://files.pythonhosted.org/packages/87/c6/53da25344e3e3a9c01095a89f16dbcda021c609ddb42dd6d7c0528236fb2/atomicwrites-1.4.1.tar.gz
about:
  home: https://github.com/untitaker/python-atomicwrites
  PyPI: https://pypi.org/project/atomicwrites
  summary: Atomic file writes.
  license: MIT
