Fix `get_cmap()` deprecation.

[ehpor]

Fixes https://github.com/ehpor/hcipy/issues/204

[codecov[bot]]

Codecov Report

Merging #207 (df86ffb) into master (78947e9) will decrease coverage by 0.05%. The diff coverage is 44.00%.

@@            Coverage Diff             @@
##           master     #207      +/-   ##
==========================================
- Coverage   81.82%   81.78%   -0.05%     
==========================================
  Files          97       97              
  Lines        7308     7319      +11     
==========================================
+ Hits         5980     5986       +6     
- Misses       1328     1333       +5     

Files	Coverage Δ
hcipy/plotting/field.py	80.50% <100.00%> (+0.50%)	:arrow_up:
hcipy/plotting/animation.py	71.15% <30.00%> (-2.01%)	:arrow_down:

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