Fixed deprecated DivergingNorm -> TwoSlopeNorm

Jan Caron requested to merge caron/empyre:matplotlib-fix into master

Needed to raise matplotlib version to 3.2!

@weber, @clausen

I hope this fixes the matplotlib error with the DivergingNorm. Let me know if there's anything else missing. The pipeline (with matplotlib 3.2 succeeded on my fork and should also do on the main repo. If it does, I'll publish another release (0.3.1) that you can use for LiberTEM.

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