Enable old Boltzmann key with DeprecationWarning

[xhgchen]

Fixes #4229

Changes made in this Pull Request:

• Provide old 'Boltzman_constant' key in subclassed dict and wrap __getitem__ with DeprecationWarning
• Add test for old 'Boltzman_constant' key
• Add test for deprecation warning with old 'Boltzman_constant' key

PR Checklist

• Tests?
• CHANGELOG updated?
• Issue raised/referenced?

Developers certificate of origin

• I certify that this contribution is covered by the LGPLv2.1+ license as defined in our LICENSE and adheres to the Developer Certificate of Origin.

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📚 Documentation preview 📚: https://mdanalysis--4230.org.readthedocs.build/en/4230/

[github-actions]

Linter Bot Results:

Hi @xhgchen! Thanks for making this PR. We linted your code and found the following:

Some issues were found with the formatting of your code.

Code Location	Outcome
main package	⚠️ Possible failure
testsuite	⚠️ Possible failure

Please have a look at the darker-main-code and darker-test-code steps here for more details: https://github.com/MDAnalysis/mdanalysis/actions/runs/5815028750/job/15765827233

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Please note: The black linter is purely informational, you can safely ignore these outcomes if there are no flake8 failures!

[IAlibay]

changelog entry please

[xhgchen]

Done! Was just in the middle of doing it as you replied 😅

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (2bf55dd) 93.62% compared to head (874a8df) 93.62%.
Report is 2 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #4230   +/-   ##
========================================
  Coverage    93.62%   93.62%           
========================================
  Files          193      193           
  Lines        25295    25304    +9     
  Branches      4063     4064    +1     
========================================
+ Hits         23683    23692    +9     
  Misses        1096     1096           
  Partials       516      516           

Files Changed	Coverage Δ
package/MDAnalysis/units.py	100.00% <100.00%> (ø)

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[xhgchen]

Some issues are appearing with the CI and Codecov, but I don't think they're from my PR.

[hmacdope]

Thanks for sorting this out @xhgchen :)