By splitting the implementation into `Scheduler` and `Pool` types,
the user-facing Scheduler type is now free of reference cycles:

- no more `Scheduler.finish()` method to break reference cycles
- automatic join when no references to the scheduler remain

- Add Scheduler.join() method
- Allow for multiple root tasks
- Allow for no task at all

This sole hunk has been extracted from:
https://github.com/abilian/cythonplus-sandbox/commit/a3743de8437e482720c1f441d5a019f18a390df1.patchCo-authored-by: Jerome Dumonteil <jd@abilian.com>
Co-authored-by: Xavier Thompson <xavier.thompson@nexedi.com>

This code has been extracted from:
https://lab.nexedi.com/jjerphan/kdtree/tree/228d5de5a53040f1a46280a6a09576a62c541d4a

This code has originally be written for and vendored in:
https://lab.nexedi.com/xavier_thompson/scan-filesystem