Fix for python 3.8

Workaround for the exception

Exception ignored in: <function Pool.__del__ at 0x7faaa5bc1b80>
Traceback (most recent call last):
  File "/cvmfs/sft-nightlies.cern.ch/lcg/nightlies/dev4/Tue/Python/3.9.5/x86_64-centos7-gcc10-opt/lib/python3.9/multiprocessing/pool.py", line 268, in __del__
  File "/cvmfs/sft-nightlies.cern.ch/lcg/nightlies/dev4/Tue/Python/3.9.5/x86_64-centos7-gcc10-opt/lib/python3.9/multiprocessing/queues.py", line 372, in put
AttributeError: 'NoneType' object has no attribute 'dumps'

Introduced by Python 3.8. See e.g. discussion in https://github.com/dask/dask/issues/5806