Make YamlSpecWriter.sort_keys a static method

[rly]

Motivation

Cleaning up code. Follow up to hdmf-dev/hdmf-docutils#92 (comment)

Checklist

• Did you update CHANGELOG.md with your changes?
• Does the PR clearly describe the problem and the solution?
• Have you reviewed our Contributing Guide?
• Does the PR use "Fix #XXX" notation to tell GitHub to close the relevant issue numbered XXX when the PR is merged?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.68%. Comparing base (8edb6d1) to head (7dfa41e).
⚠️ Report is 11 commits behind head on dev.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev    #1274   +/-   ##
=======================================
  Coverage   91.68%   91.68%           
=======================================
  Files          42       42           
  Lines        9598     9599    +1     
  Branches     1933     1933           
=======================================
+ Hits         8800     8801    +1     
  Misses        519      519           
  Partials      279      279           

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