Miscelaneous bugfixes (matplotlib & futuretests)

Created by: scarlehoff

This PR fixes #1808 (closed)

and on doing this I realized there were some problems with the latest versions of matplotlib.

1. We were using ax._get_lines.prop_cycler which is undocumented and removed in 3.7. I've changed it to the a bit-less-undocumented color = ax._get_lines.get_next_color()
2. The transformation to avoid having points on top of each other doesn't work in matplotlib 3.8, hence the pinning of the version in the conda recipe https://github.com/NNPDF/nnpdf/blob/f553482181a9d785a2386ae29c44efc765b74d7f/validphys2/src/validphys/plotutils.py#L289