AttributeError: module 'multiprocessing.dummy' has no attribute 'cpu_count'

Duplicity version: 0.8.10
Python: 3.8
OS: Arch Linux 64-bit
Type of target filesystem: S3

Command:

duplicity -v9 --full-if-older-than 1M --encrypt-sign-key <redacted> --s3-use-new-style \
  --s3-use-ia --s3-use-multiprocessing /home/rob s3+http://<redacted>/home/rob

Getting the following exception when starting a backup to S3. Full log attached.

 Traceback (innermost last):
   File "/bin/duplicity", line 100, in <module>
     with_tempdir(main)
   File "/bin/duplicity", line 86, in with_tempdir
     fn()
   File "/lib/python3.8/site-packages/duplicity/dup_main.py", line 1525, in main
     action = commandline.ProcessCommandLine(sys.argv[1:])
   File "/lib/python3.8/site-packages/duplicity/commandline.py", line 1187, in ProcessCommandLine
     backup, local_pathname = set_backend(args[0], args[1])
   File "/lib/python3.8/site-packages/duplicity/commandline.py", line 1058, in set_backend
     globals.backend = backend.get_backend(bend)
   File "/lib/python3.8/site-packages/duplicity/backend.py", line 225, in get_backend
     obj = get_backend_object(url_string)
   File "/lib/python3.8/site-packages/duplicity/backend.py", line 211, in get_backend_object
     return factory(pu)
   File "/lib/python3.8/site-packages/duplicity/backends/_boto_multi.py", line 99, in __init__
     self._setup_pool()
   File "/lib/python3.8/site-packages/duplicity/backends/_boto_multi.py", line 104, in _setup_pool
     number_of_procs = multiprocessing.cpu_count()
  AttributeError: module 'multiprocessing.dummy' has no attribute 'cpu_count'
 Traceback (most recent call last):
   File "/usr/local/bin/duplicity-unattended", line 136, in <module>
     main()
   File "/usr/local/bin/duplicity-unattended", line 69, in main
     run(backup_cmd, check=True, env=env)
   File "/usr/lib/python3.8/subprocess.py", line 512, in run
     raise CalledProcessError(retcode, process.args,