Fix ComboBox with unhashable choice data

[tlambert03]

This allows the "data" for one of the choices in a categorical widget to be unhashable.
(for instance combo.choices = (('a', [1,2,3]), ('b', [3,4,5)))

This is necessary to implement the napari feature requested by @sofroniewn here

[codecov]

Codecov Report

Merging #89 (d7ed4f2) into master (ce7346a) will increase coverage by 0.26%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #89      +/-   ##
==========================================
+ Coverage   86.64%   86.90%   +0.26%     
==========================================
  Files          15       15              
  Lines        1670     1673       +3     
==========================================
+ Hits         1447     1454       +7     
+ Misses        223      219       -4     

Impacted Files	Coverage Δ
magicgui/backends/_qtpy/widgets.py	84.04% <100.00%> (+1.76%)	⬆️

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[sofroniewn]

Thanks @tlambert03. LGTM!