Bug#954403: joblib: configuration error installing python3-joblib

[Graham Inggs]

As can be seen in the autopkgtests of circlator [1] and spades [2],
this started happening again with the upload of python3-defaults
3.9.0-3, which switched the default Python version from 3.8 to 3.9.

It can also be seen that the autopkgtests of both circlator and spades
previously started passing again on 2020-04-08, as soon as
python3-defaults 3.8.2-3 was uploaded, removing Python 3.7 as a
supported version.


[1] https://ci.debian.net/packages/c/circlator/testing/amd64/
[2] https://ci.debian.net/packages/s/spades/testing/amd64/