community/py3-matplotlib: add dependency py3-packaging and take maintainership

py3-packaging is listed as an explicit dependency of py3-matplotlib in the setup.py, and not having py3-packaging installed causes the FreeCAD tests to fail due to py3-matplotlib not being able to find it. I'm also interested in taking maintainership of py3-matplotlib as it currently doesn't have a maintainer and it is a dependency of FreeCAD as well as Jupyter (!29824 (merged)).