Change bins parameter in imghist to auto

[SarthakJariwala]

bins parameter changed to None (which is auto) in imghist

[codecov-commenter]

Codecov Report

Merging #16 into master will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #16      +/-   ##
==========================================
+ Coverage   97.23%   97.25%   +0.02%     
==========================================
  Files           6        6              
  Lines         253      255       +2     
  Branches       51       52       +1     
==========================================
+ Hits          246      248       +2     
  Misses          3        3              
  Partials        4        4              

Impacted Files	Coverage Δ
src/seaborn_image/_general.py	100.00% <100.00%> (ø)

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