'JupyterLab' as an optional dependency of Pyxel

Currently, when a user installs Pyxel from PyPi, JupyterLab is automatically installed.

Unfortunately, we shouldn't force the installation of JupyterLab because:

• Pyxel can run without JupyterLab
• Maybe the user wants to use Jupyter Notebook, Jupyter Hub, [Retrolab](https://blog.jupyter.org/retrolab-a-jupyterlab-distribution-with-a-retro-look-and-feel-8096b8b223d0], JupyterLite or directly in VSCode or JetBrains Datalore.

Therefore, JupyterLab should be an optional dependency for Pyxel.

Example:

Installation without Jupyter lab
$ pip install pyxel-sim
 
With Jupyter lab
$ pip install pyxel-sim[jupyter]
 
With everything (poppy, pygmo...)
$ pip install pyxel-sim[all]