SPEC0 - support Python 3.12 to 3.14

[trexfeathers]

🚀 Pull Request

Description

Scientific Python - SPEC 0 — Minimum Supported Dependencies

Closes #6775

Slight hiccup:

• We aim to provide Python 3.14 compatibility in an Iris 3.14.1 patch release. See 🦌 Iris 3.14.x #6626 (comment)
• It would be inappropriate to change the our supported versions in a patch release

So here are my proposed steps:

• Put up this draft pull request, which includes Python 3.14 test coverage, thus demonstrating that 719320c successfully provides Python 3.14 compatibility.
• Apply 719320c to the v3.14.x release branch, which will give the 3.14.1 release compatibility with Python 3.14.
• Do a merge-back from v3.14.x to main; this needn't wait for the release itself.
• Rebase this pull request onto the latest main.
• If there has been a delay: re-resolve the lock-files in this PR.
• What's New entry.
• Take this PR out of draft.

The above pattern will avoid Git conflicts and maintain maximum integrity in our Git history, avoiding situations where a change has two different SHAs depending on which branch it is on.

Terminology

• Compatibility: works with Python 3.14
• Support: is tested against Python 3.14

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Consult Iris pull request check list

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Add any of the below labels to trigger actions on this PR:

• benchmark_this Request that this pull request be benchmarked to check if it introduces performance shifts

[scitools-ci]

• Please see: SciTools/.github#234

SciTools/.github#234 (comment)

[trexfeathers]

The failures appear to come from Python 3.14 incompatibilities in dependencies:

• PyVista: importing pyvista with python 3.14 raises AttributeError pyvista/pyvista#8112
• Mule, which is not an explicit dependency but supports benchmark data generation

These prevent all of Iris' CI running correctly. However the vast majority of tests have run, and have passed, including many tests which exercise DimCoord.__deepcopy__() - the original problem.

I believe we are therefore able to provide Python 3.14 compatibility, although we're not yet able to provide support due to these CI problems preventing full coverage.

[trexfeathers]

• Mule, which is not an explicit dependency but supports benchmark data generation

#6856