esa/pagmo2: pagmo 2.12.0

This new pagmo/pygmo release feature 2 brand new algorithms (NSPSO and MHACO), and an important feature addition to GPSO, which can now use the batch fitness evaluation scheme. On the python side, there are some breaking changes in the API of ipyparallel_island, which, on the plus side, should take care of various issues that we have been intermittently experiencing on our CI setup. This release features also the usual build system/doc improvements/maintenance and assortment of small fixes. As a public service announcement, this will be the last release of pygmo to officially support Python 2.7, which, as you may know, is reaching its end of life very soon (https://pythonclock.org/). We do not plan to actively go and remove existing Python 2 support code within pygmo (at least not initially), but we will simply stop ensuring that pygmo does work on Python 2, and new pygmo code will target Python 3 exclusively. We will also stop producing binary packages for Python 2, both pip and conda (please note that conda-forge will also remove Python 2 support soon). If anyone in the pygmo community wants to take charge of maintaining Python 2 support, we are open to discussions in this sense (please come over to our gitter channel: https://gitter.im/pagmo2/Lobby). The full changelog, as usual, is available here: https://esa.github.io/pagmo2/changelog.html