Add Cython classifier to package metadata

[djhoese]

A request was recently posted on the python mailing list about including Cython as the language of your package in the setup.py metadata. This PR adds that classifier.

• Closes #xxxx
• Tests added
• Tests passed
• Passes git diff origin/main **/*py | flake8 --diff
• Fully documented

[codecov]

Codecov Report

Merging #427 (b82a750) into main (d6cc9e4) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #427   +/-   ##
=======================================
  Coverage   93.95%   93.95%           
=======================================
  Files          65       65           
  Lines       11252    11252           
=======================================
  Hits        10572    10572           
  Misses        680      680           

Flag	Coverage Δ
unittests	93.95% <ø> (ø)

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[coveralls]

Coverage remained the same at 93.78% when pulling b82a750 on djhoese:add-cython-classifier into d6cc9e4 on pytroll:main.

[mraspaud]

LGTM