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: retrying
  version: 1.3.4
  top-level:
    - retrying
source:
  sha256: 345da8c5765bd982b1d1915deb9102fd3d1f7ad16bd84a9700b85f64d24e8f3e
  url: https://files.pythonhosted.org/packages/ce/70/15ce8551d65b324e18c5aa6ef6998880f21ead51ebe5ed743c0950d7d9dd/retrying-1.3.4.tar.gz
requirements:
  run:
    - six
about:
  home: https://github.com/rholder/retrying
  PyPI: https://pypi.org/project/retrying
  summary: Retrying
  license: Apache 2.0

package:
  name: retrying
  version: 1.3.4
  top-level:
    - retrying
source:
  sha256: 345da8c5765bd982b1d1915deb9102fd3d1f7ad16bd84a9700b85f64d24e8f3e
  url: https://files.pythonhosted.org/packages/ce/70/15ce8551d65b324e18c5aa6ef6998880f21ead51ebe5ed743c0950d7d9dd/retrying-1.3.4.tar.gz
requirements:
  run:
    - six
about:
  home: https://github.com/rholder/retrying
  PyPI: https://pypi.org/project/retrying
  summary: Retrying
  license: Apache 2.0
