bug#58993: Duplicity BackendException: No module named b2sdk

[Cayetano Santos]

Context:

Using latest guix as a package manager under a foreign up to date 
archlinux distribution.

Problem:

When using duplicity to backup to a sftp server, I get this error 
message

   BackendException: Could not initialize backend: No module 
   named 'b2sdk'

As a reference, see https://issues.guix.gnu.org/49979.