Enable python 3.9 and mamba build
Created by: scarlehoff
As per https://github.com/NNPDF/nnpdf/issues/1458#issuecomment-958807144
If the workflow works I'll enable also 3.8 so that we have both for the time being.
Created by: scarlehoff
As per https://github.com/NNPDF/nnpdf/issues/1458#issuecomment-958807144
If the workflow works I'll enable also 3.8 so that we have both for the time being.