revert benchmark workaround + whatsnew entry

Author: bjlittle

benchmarks/asv.conf.json

@@ -23,7 +23,7 @@
     //  * No build-time environment variables.
     //  * Is run in the same environment as the ASV install itself.
     "delegated_env_commands": [
-        "PY_VER=3.11 nox --envdir={conf_dir}/.asv/env/nox01 --session=tests --install-only --no-error-on-external-run --verbose"
+        "PY_VER=3.12 nox --envdir={conf_dir}/.asv/env/nox01 --session=tests --install-only --no-error-on-external-run --verbose"
     ],
     // The parent directory of the above environment.
     //  The most recently modified environment in the directory will be used.

benchmarks/bm_runner.py

@@ -67,7 +67,7 @@ def _check_requirements(package: str) -> None:
 
 def _prep_data_gen_env() -> None:
     """Create or access a separate, unchanging environment for generating test data."""
-    python_version = "3.11"
+    python_version = "3.12"
     data_gen_var = "DATA_GEN_PYTHON"
     if data_gen_var in environ:
         echo("Using existing data generation environment.")
@@ -87,7 +87,7 @@ def _prep_data_gen_env() -> None:
         # Find the environment built above, set it to be the data generation
         #  environment.
         data_gen_python = next(
-            (ROOT_DIR / ".nox").rglob("tests*/bin/python3.12")
+            (ROOT_DIR / ".nox").rglob(f"tests*/bin/python{python_version}")
         ).resolve()
         environ[data_gen_var] = str(data_gen_python)
 

docs/src/whatsnew/latest.rst

@@ -74,7 +74,8 @@ This document explains the changes made to Iris for this release
 🔗 Dependencies
 ===============
 
-#. N/A
+#. `@bjlittle`_ dropped support for ``py39`` and adopted support for ``py312`` as per
+   the `NEP-29`_ schedule. (:pull:`5894`)
 
 
 📚 Documentation
@@ -100,6 +101,7 @@ This document explains the changes made to Iris for this release
 
 #. `@jfrost-mo`_ enabled colour output for pytest on GitHub Actions. (:pull:`5895`)
 
+
 .. comment
     Whatsnew author names (@github name) in alphabetical order. Note that,
     core dev names are automatically included by the common_links.inc:
@@ -115,3 +117,4 @@ This document explains the changes made to Iris for this release
 .. _NPY002: https://docs.astral.sh/ruff/rules/numpy-legacy-random/
 .. _numpydoc validation: https://numpydoc.readthedocs.io/en/latest/validation.html#
 .. _Dask version 2024.2.1: https://docs.dask.org/en/stable/changelog.html#v2024-2-1
+.. _NEP-29: https://numpy.org/neps/nep-0029-deprecation_policy.html#drop-schedule