Six and eight color Petroff color cycles

The six and eight color accessible Petroff color cycles are named 'petroff6' and

'petroff8'.

They compliment the existing 'petroff10' color cycle, added in `Matplotlib 3.10.0`_

For more details see

`Petroff, M. A.: "Accessible Color Sequences for Data Visualization"

<https://arxiv.org/abs/2107.02270>`_.

To load these color cycles in place of the default::

or

import matplotlib.pyplot as plt

plt.style.use('petroff8')

.. _Matplotlib 3.10.0: https://matplotlib.org/stable/users/prev_whats_new/whats_new_3.10.0.html#new-more-accessible-color-cycle