Require Python >=3.12

[lewisjared]

Description

Raises the minimum supported Python version to 3.12 and modernises the codebase for it. Two commits:

• chore: require Python >=3.12 — requires-python across all packages, trove classifiers, .python-version, README badges, install docs, and the CI matrices now target 3.12. uv.lock is regenerated on 3.12, which drops the <3.12 backport packages (async-timeout, importlib-metadata, tomli, zipp, overrides).
• style: modernise syntax for Python 3.12 — raises the ruff target-version to py312 and applies the resulting pyupgrade fixes: datetime.UTC, Self/StrEnum imports, PEP 695 generic syntax, and removal of dead sys.version_info compatibility blocks.

Motivation: newer ilamb3 (>=2026.4.20, and the in-flight amoc-rapid trim_time fix) requires Python >=3.12.

Verified locally: ruff check clean, mypy clean (129 files), the CLI/config/constraints suites pass (452 passed), and the full workspace collects (2331 tests) and builds on 3.12.

Checklist

Please confirm that this pull request has done the following:

• Tests added
• Documentation added (where applicable)
• Changelog item added to changelog/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Flag	Coverage Δ
core	92.58% <100.00%> (+0.01%)	⬆️
providers	91.82% <ø> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...te-ref-core/src/climate_ref_core/cmip6_to_cmip7.py	94.50% <100.00%> (ø)
...imate-ref-core/src/climate_ref_core/constraints.py	98.57% <100.00%> (+0.90%)	⬆️
...ate-ref-core/src/climate_ref_core/pycmec/metric.py	98.21% <100.00%> (-0.01%)	⬇️
...ate-ref-core/src/climate_ref_core/pycmec/output.py	98.68% <100.00%> (-0.02%)	⬇️
...ges/climate-ref/src/climate_ref/_config_helpers.py	89.61% <100.00%> (ø)
...ckages/climate-ref/src/climate_ref/cli/__init__.py	96.96% <100.00%> (ø)
...ages/climate-ref/src/climate_ref/cli/executions.py	94.88% <100.00%> (ø)
...kages/climate-ref/src/climate_ref/cli/providers.py	98.78% <100.00%> (ø)
...s/climate-ref/src/climate_ref/executor/fragment.py	86.15% <100.00%> (ø)
packages/climate-ref/src/climate_ref/solver.py	98.90% <100.00%> (ø)

... and 1 file with indirect coverage changes

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