Jupyter with Cython+

Add Cython+ in the Jupyter SR to allow compiling cython+ code in a notebook, as described in the Cython documentation:

• Add gcc-10.2 in component/gcc (this was ultimately not used yet)
• Add component/cythonplus and software/cythonplus-dev
• Remove software/cython-test, the same functionality is offered by software/cythonplus-dev
• Modify software/jupyter to add gcc in the PATH and a cython+ repository in the PYTHONPATH