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: gensim
  version: 4.3.0
  top-level:
    - gensim
source:
  url: https://github.com/RaRe-Technologies/gensim/archive/refs/tags/4.3.0.tar.gz
  sha256: 14f5f46425249ad415f40e6a87cf6b8e19a8649a1805e725947243495b600977
requirements:
  run:
    - numpy
    - scipy
    - six
    - smart_open
about:
  home: http://radimrehurek.com/gensim
  PyPI: https://pypi.org/project/gensim
  summary: Python framework for fast Vector Space Modelling
  license: LGPL-2.1-only

package:
  name: gensim
  version: 4.3.0
  top-level:
    - gensim
source:
  url: https://github.com/RaRe-Technologies/gensim/archive/refs/tags/4.3.0.tar.gz
  sha256: 14f5f46425249ad415f40e6a87cf6b8e19a8649a1805e725947243495b600977
requirements:
  run:
    - numpy
    - scipy
    - six
    - smart_open
about:
  home: http://radimrehurek.com/gensim
  PyPI: https://pypi.org/project/gensim
  summary: Python framework for fast Vector Space Modelling
  license: LGPL-2.1-only
