add `matplotlib` as a dependency

xref https://github.com/pysal/inequality/pull/75#pullrequestreview-2275036652

Since we are using `matplotib` in `schutz` and [`pen`](https://github.com/pysal/inequality/blob/03ca5619d50839be8d0a68a427027530d971dcdd/inequality/pen.py#L47), perhaps it makes sense to just go ahead and make it a requirement?


