DEP: drop cblind as a hard dependency

[neutrinoceros]

A natural follow up to #377

[volodia99]

Not sure what will happen in tests/test_plotting.py when calling cblind colormap "cb.rainbow" ?

[neutrinoceros]

test_colormap_extensions_integration should be the only test that actually relies on cblind (as well as other cmap packages), and it's written in a way that each part is simply skipped if the corresponding package isn't available at runtime.

[neutrinoceros]

(test_colormap_extensions_missing_package also checks that the we get the expected behavior when a package is not installed)