CI: add generation of test coverage charts

[GreyCat]

As suggested in #146, we might benefit from generating our test coverage charts like this one:

right in the CI. Shouldn't be that hard to generate and keep them up to date automatically.

[GreyCat]

New modular CI architecture actually makes this generation more complicated, as we can't rely on GraphViz being available at every CI which runs the tests.

Moving to v0.10, as this should not block new release per se.