Release v3.9.0

[glatterf42]

Release v3.9.0 :)

How to review

• Read the diff and note that the CI checks all pass.
• In particular, check that the 'publish' workflow passes.
• Check that the rc version is available here

PR checklist

• [x] Continuous integration checks all ✅
• ~[ ] Add or expand tests; coverage checks both ✅~ Just releasing.
• ~[ ] Add, expand, or update documentation.~ Just releasing.
• [x] Update release notes.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 98.8%. Comparing base (f08447f) to head (8059d04). Report is 16 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #533 +/- ## ======================================= - Coverage 98.8% 98.8% -0.1% ======================================= Files 44 44 Lines 4804 4796 -8 ======================================= - Hits 4747 4739 -8 Misses 57 57 ``` [see 2 files with indirect coverage changes](https://app.codecov.io/gh/iiasa/ixmp/pull/533/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=iiasa)