Adjust packaging

[khaeru]

I checked the results of python -m build after merging #159 and found that both the .tar.gz and .whl outputs were about 10 MB. This was because the entire /doc/ directory—including many images added by that other PR—was included in both files.

This PR adjusts our setuptools config so that the /doc/ directory is:

• included in the .tar.gz source distribution,
• excluded from the .whl distribution.

This follows the practice of coverage and several other packages I looked at. It also ensures that users who install from .whl (with pip) don't wind up with an extra directory named site-packages/doc/ that contains the message-ix-models docs. (AFAIK, distributing these 'dangling' directories outside the ones strictly related to the package is bad practice.)

With these changes, the .whl file is also nice and slim at under 600 kB.

How to review

Note that the CI checks all pass.

PR checklist

• Continuous integration checks all ✅
• Add or expand tests; coverage checks both ✅
• Add, expand, or update documentation. N/A, packaging changes only
• Update doc/whatsnew.

[glatterf42]

Looks good and useful to me once checks pass, thanks :)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.7%. Comparing base (a01f61a) to head (1b43524).
Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #182   +/-   ##
=====================================
  Coverage   76.7%   76.7%           
=====================================
  Files        112     112           
  Lines       7154    7154           
=====================================
  Hits        5494    5494           
  Misses      1660    1660