include max value in pcolormesh output when levels are requested

[JJFlorian]

• [x] Closes #10911

Not the most elegant solution, but it works and I couldn't figure out another way of solving this for now. The test_discrete_colormap_list_of_levels test fails now, but I would say that it is safe to use numpy's assert_array_almost_equal.

If its not, I'll try to find another solution.

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