Change DEFs to cdef int constants

[hmacdope]

Fixes #4237

Changes made in this Pull Request:

• Changes cython DEFs to be integer constants due to deprecation of DEF in cython 3.0+

PR Checklist

• 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--4246.org.readthedocs.build/en/4246/

[github-actions]

Linter Bot Results:

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

There are currently no issues detected! 🎉

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: +0.01% 🎉

Comparison is base (212bf03) 93.40% compared to head (849fd09) 93.41%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #4246      +/-   ##
===========================================
+ Coverage    93.40%   93.41%   +0.01%     
===========================================
  Files          169      183      +14     
  Lines        22202    23307    +1105     
  Branches      4064     4064              
===========================================
+ Hits         20738    21773    +1035     
- Misses         948     1018      +70     
  Partials       516      516              

see 14 files with indirect coverage changes

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