use new approach in dune-common for handling python bindings

fix ci

pep 42: use implicit namespace packages which make it possible to combine subpackages using editable (source) and non editable (pypi)

only use a cmake install a python module in the SKBUILD setting - for an install from build dir pip will take take of this since it is part of the setup.py file

add a stopgap solution unti ci repo up and running again

Update requirements.

add dune packages to requires in setup.py.in

[testing]

only add dune modules that have been found as requirements in setup.py.in

removed setup.py.in since the default is provided in dune-common

adapt ci yml file to python bindings on by default

disable python bindings for clang 5 (ubuntu 18) since no numpy matching numpy version can be installed from the gitlab registry

[bug fix python bindings]
fix an issue with 'entry' being used in implicit build mode but ready==build