silx.gui: Fixed matplotlib 3.3.0rc1 deprecation warnings

[t20100]

This PR avoids deprecation warnings raised by matplotlib 3.3.0rc1 during tests.

closes #3144

[vallsv]

Was it deprecated for a long time?

        if _matplotlib_colormaps is not None:
            colormaps.update(_matplotlib_colormaps())
        elif _matplotlib_cm is not None:
            colormaps.update(_matplotlib_cm.cmap_d.keys())

Else it is maybe better to do that?

[t20100]

Even without the deprecation, matplotlib.pyplot.colormaps is what should be used, it's been there for a very long while, see https://github.com/matplotlib/matplotlib/blame/master/lib/matplotlib/pyplot.py#L1874

[vallsv]

SOunds good then