pydot tests are skipped

[ferrine]

Description

As discovered here

SKIPPED [1] tests/d3viz/test_d3viz.py:17: pydot not available: Install the python package pydot or pydot-ng. Install graphviz.
SKIPPED [1] tests/d3viz/test_formatting.py:9: pydot not available: Install the python package pydot or pydot-ng. Install graphviz.