Add Labels.skeleton convenience attribute

[talmo]

Implements #45.

[codecov]

Codecov Report

Merging #54 (78dde4b) into main (8b444b1) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #54      +/-   ##
==========================================
+ Coverage   94.39%   94.43%   +0.03%     
==========================================
  Files          12       12              
  Lines        1053     1060       +7     
==========================================
+ Hits          994     1001       +7     
  Misses         59       59              

Impacted Files	Coverage Δ
sleap_io/model/labels.py	100.00% <100.00%> (ø)
sleap_io/model/skeleton.py	100.00% <100.00%> (ø)

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