Properly find and activate python

The cmake function add_dune_pythonlibs_flags did set the HAVE_PYTHON flag unconditionally. Because it was not found in my setup this lead to compiler errors like missing include <Python.h>. With this MR, these flags are only added if Python is actually found somehow. Also it adds the actual call to find_package(Python3...) that was missing before. Note that the Python3_FOUND is only set if python was found in the current scope. Since you cannot rely on the way other module might have searched for python and might have set this variable, it is better to just search for Python again.

Note that the pipeline currently fails, since no runner is picked up for my fork branch

This might be, because there are no runner tags set in the gitlab-ci.yml file.

But there are runners associated to this project which are marked to run untagged jobs, too. This used to work. I also noticed this behavior elsewhere. Was there maybe a change in gitlab that deactivates a runner for non-tagged jobs (despite the option being activated) if there are also tags given for the runner?

Hm, we once explicitly dropped test for python in dune-fufem, because it interfered with the one in dune-common. Back then the advice was that a clean solution is to not run the test here again, cf. core/dune-common#235 (closed).

mentioned in commit 8da9646d

mentioned in merge request !260 (merged)

closed