feat: Add ratio plot support

[matthewfeickert]

Resolves #22

* Update minimum required version of hist to v2.3.0
   - c.f. https://github.com/scikit-hep/hist/pull/161
* Add stack_ratio_plot to heputils.plot API
   - Will be revised in future release for better API
* Update dev-example notebook with examples of ratio plots

[codecov]

Codecov Report

Merging #60 (c4b2681) into main (5f8aee4) will decrease coverage by 3.16%.
The diff coverage is 3.22%.

@@            Coverage Diff             @@
##             main      #60      +/-   ##
==========================================
- Coverage   27.96%   24.79%   -3.17%     
==========================================
  Files           7        7              
  Lines         211      242      +31     
  Branches       39       42       +3     
==========================================
+ Hits           59       60       +1     
- Misses        152      182      +30     

Flag	Coverage Δ
unittests	24.79% <3.22%> (-3.17%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/heputils/plot.py	16.58% <3.22%> (-2.31%)	⬇️

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