Fix matplotlib 3.9+ incompatibility in pymnet_style visualization

[Copilot]

matplotlib.cm.get_cmap() was removed in matplotlib 3.9, breaking the draw_multilayer_pymnet visualization and causing the run-examples (3.11) CI job to fail.

Changes

• py3plex/visualization/pymnet_style.py: Replace all 3 calls to plt.cm.get_cmap(name) with mpl.colormaps[name] — the modern API available since matplotlib 3.5, consistent with the existing import matplotlib as mpl in the file.

# Before (removed in matplotlib 3.9)
cmap = plt.cm.get_cmap('tab10')

# After
cmap = mpl.colormaps['tab10']

Affected code paths: node coloring by layer (×2) and by degree (×1) inside _get_node_colors.