VertexCover.__plot__ mark_groups argument does not behave correctly and can cause a crash

[Aethor]

Describe the bug
VertexCover.__plot__ mark_groups specifies in the doc that it accepts a "dict mapping cluster indices or tuples of vertex indices to color names". However, giving such a dict results in an AttributeError:

AttributeError: 'dict' object has no attribute 'iteritems'

This is because mark_groups is parsed using igraph.clustering._handle_mark_groups_arg_for_clustering, which uses iteritems that was removed in Python3. Using items instead fixes the issues. I'm happy to provide a PR if needed.

To reproduce

import igraph as ig
import matplotlib.pyplot as plt


g = ig.Graph(edges=[[0, 1], [2, 3]])
clusters = [[0, 1], [2, 3]]
cover = ig.VertexCover(g, clusters)
fig, ax = plt.subplots()
ig.plot(
    cover,
    target=ax,
    mark_groups={0: "red", 1: "green"},
)
plt.show()

Note that there is a workaround:

import igraph as ig
import matplotlib.pyplot as plt


g = ig.Graph(edges=[[0, 1], [2, 3]])
clusters = [[0, 1], [2, 3]]
cover = ig.VertexCover(g, clusters)
fig, ax = plt.subplots()
ig.plot(
    cover,
    target=ax,
    mark_groups={0: "red", 1: "green"}.items(), # <= ensure correct parsingof mark_groups
)
plt.show()

Version information
0.11.3, obtained from pypi using pip. (but 0.11.6 should also be subject to the bug as the related code is the same)

[ntamas]

Thanks for the report, this is now fixed in the source.