Matplotlib warnings in seaborn example: "Trying to register the cmap which already exists"

[lesteve]

Seen in scikit-learn: scikit-learn/scikit-learn#23828

/home/circleci/miniconda/envs/testenv/lib/python3.9/site-packages/seaborn/cm.py:1582: UserWarning: Trying to register the cmap 'rocket' which already exists.
  mpl_cm.register_cmap(_name, _cmap)

Googling for the error does turn up other projects that are affected. I think this is because the reset_seaborn logic by removing the seaborn modules from sys.modules, triggers colormaps registration multiple times if seaborn is used in multiple examples. It seems like using seaborn.reset_defaults could be used instead the sys.modules removal.

I have opened #969 to use seaborn.reset_defaults and see what happens.