Make __cosmology_namespace__ a property

[nstarman]

Make __cosmology_namespace__ a property.

PR Checklist

• Give a detailed description of the PR above.
• Document changes in the CHANGES.rst file. See existing changelog for examples.
• Add tests, if applicable, to ensure code coverage never decreases.
• Make sure the docs are up to date, if applicable, particularly the docstrings and RST files in docs folder.
• Ensure linear history by rebasing, when requested by the maintainer.

[codecov-commenter]

Codecov Report

Merging #10 (a823243) into main (7853add) will increase coverage by 0.97%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #10      +/-   ##
==========================================
+ Coverage   79.64%   80.61%   +0.97%     
==========================================
  Files           4        4              
  Lines         226      227       +1     
==========================================
+ Hits          180      183       +3     
+ Misses         46       44       -2     

Impacted Files	Coverage Δ
src/cosmology/compat/classy/_core.py	88.88% <100.00%> (+8.11%)	⬆️