Bump pineappl version

Created by: Radonirinaunimi

Now that pineappl v0.8.1 is out we can update here. In principle, these changes are also introduced in https://github.com/NNPDF/nnpdf/pull/2110 but given that this is needed to make pineko work (https://github.com/NNPDF/pineko/pull/181) it would be better to have this in master first.

Tests relying on conda are still failing because of https://github.com/NNPDF/pineappl/issues/300.