Ensure compatibility with pandas CoW

[glatterf42]

Closes #804. This PR temporarily enables the pandas Copy-on-Write behaviour that will become standard with pandas 3.0. With this, I ran all tests locally to study the warnings we would get from this: only two files were affected, add_year/__init__ and macro. Then, I fixed all of these warnings as suggested in the migration guide, so we might be good with this repo (and all underlying ixmp functions called from the test suite here) :)

How to review

• Read the diff and note that the CI checks all pass.
• Study CI output to see if any pandas Warnings wrt CoW came up that were skipped locally.
• Fix new Warnings (if any).
• Remove the TEMPORARY commit to avoid enabling pandas CoW for all users already.

PR checklist

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

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 71.59091% with 25 lines in your changes are missing coverage. Please review.

Project coverage is 95.6%. Comparing base (4207f5c) to head (4913c24). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #842 +/- ## ======================================= + Coverage 95.4% 95.6% +0.1% ======================================= Files 46 46 Lines 4354 4339 -15 ======================================= - Hits 4156 4149 -7 + Misses 198 190 -8 ``` | [Files](https://app.codecov.io/gh/iiasa/message_ix/pull/842?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=iiasa) | Coverage Δ | | |---|---|---| | [message\_ix/macro.py](https://app.codecov.io/gh/iiasa/message_ix/pull/842?src=pr&el=tree&filepath=message_ix%2Fmacro.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=iiasa#diff-bWVzc2FnZV9peC9tYWNyby5weQ==) | `96.7% <100.0%> (+<0.1%)` | :arrow_up: | | [message\_ix/tools/add\_year/\_\_init\_\_.py](https://app.codecov.io/gh/iiasa/message_ix/pull/842?src=pr&el=tree&filepath=message_ix%2Ftools%2Fadd_year%2F__init__.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=iiasa#diff-bWVzc2FnZV9peC90b29scy9hZGRfeWVhci9fX2luaXRfXy5weQ==) | `66.8% <70.9%> (+0.6%)` | :arrow_up: |

[glatterf42]

Not sure why codecov/patch registers a diff hit, we test the same functions as before. So I'm fine with the hit if that comes from additional lines that reduce complexity. I'll rebase to exclude the TEMPORARY commit and merge once the tests pass (again).