Remove version restriction on billiard

Fixes this crash:

Traceback (most recent call last):                                                                                                                                        
  File "/home/stsp/.virtualenvs/swh/lib/python3.11/site-packages/_pytest/config/__init__.py", line 781, in import_plugin                                                  
    __import__(importspec)                                                                                                                                                
  File "/home/stsp/.virtualenvs/swh/lib/python3.11/site-packages/_pytest/assertion/rewrite.py", line 178, in exec_module
    exec(co, module.__dict__)                                                                                                                                             
  File "/home/stsp/swh-environment/swh-scheduler/swh/scheduler/pytest_plugin.py", line 9, in <module>
    from celery.contrib.testing import worker                                                                                                                             
  File "/home/stsp/.virtualenvs/swh/lib/python3.11/site-packages/celery/contrib/testing/worker.py", line 8, in <module>                                                   
    import celery.worker.consumer  # noqa                                                                                                                                 
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^                                                                                                                                         
  File "/home/stsp/.virtualenvs/swh/lib/python3.11/site-packages/celery/worker/consumer/__init__.py", line 4, in <module>                                                 
    from .consumer import Consumer                                                                                                                                        
  File "/home/stsp/.virtualenvs/swh/lib/python3.11/site-packages/celery/worker/consumer/consumer.py", line 24, in <module>                                                
    from celery.app.trace import build_tracer                                                                                                                             
  File "/home/stsp/.virtualenvs/swh/lib/python3.11/site-packages/celery/app/trace.py", line 14, in <module>                                                               
    from billiard.einfo import ExceptionInfo, ExceptionWithTraceback                                                                                                      
ImportError: cannot import name 'ExceptionWithTraceback' from 'billiard.einfo' (/home/stsp/.virtualenvs/swh/lib/python3.11/site-packages/billiard/einfo.py)