Record Python package versions in GDX files

[khaeru]

This PR provides the basic functionality for addressing iiasa/message_ix#747.

The GAMSModel class gains an attribute/parameter (record_version_packages) and a method (record_versions()); the latter is called from within the run() method. This adds a set to the Scenario that is to be solved, with entries like:

Set ixmp_version(*,*) Versions of Python packages used to generate this model /
'message_ix'.'3-7-1-dev55+g2e0a9c49', 
'ixmp'.'3-7-1-dev67+g6cab755-d20231122'
'notapackage'.'(not installed)' /;

(The list of included packages is according to record_version_packages, so can be extended by downstream packages, like message-ix, that use or subclass GAMSModel.)

How to review

Read the diff and note that the CI checks all pass.

Test together with a message_ix PR to be opened next.

PR checklist

• Continuous integration checks all ✅
• Add or expand tests; coverage checks both ✅
• Add, expand, or update documentation.
• Update release notes.

[khaeru]

@glatterf42 FYI. I'll request your review once the checklist is complete.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (41f8a96) 98.9% compared to head (36b6c16) 98.9%.

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #502   +/-   ##
=====================================
  Coverage   98.9%   98.9%           
=====================================
  Files         44      44           
  Lines       4758    4792   +34     
=====================================
+ Hits        4706    4741   +35     
+ Misses        52      51    -1     

Files	Coverage Δ
ixmp/backend/jdbc.py	97.4% <100.0%> (+0.1%)	⬆️
ixmp/model/gams.py	100.0% <100.0%> (ø)
ixmp/report/operator.py	99.0% <ø> (ø)
ixmp/tests/backend/test_jdbc.py	100.0% <100.0%> (ø)
ixmp/tests/core/test_scenario.py	100.0% <100.0%> (ø)

[glatterf42]

Looks good to me, thanks :)