Fixes irisxr 202608

[pp-mo]

Closes #255
Closes #258

[pp-mo]

Progress update 2026-09-19

Extensively rebased.
Should now work with forthcoming Iris 3.16.1, if it includes SciTools/iris#7281

Stiil incomplete, as more needed to get it working with latest xarray (>v2026.02.0).
Possibly, all that will be needed is to exclude all "unstructured" testfiles from integration.test_iris_xarray_roundtrips.test_roundtrip_ixi

TODO:

remove temporary test-against-private-branch code, in .github/workflows/ci-tests.yml

[pp-mo]

Removed test-against-new-iris .

[pp-mo]

Note:

when testing against older Iris, just 3 string-array tests fail,
all in tests/integration/test_iris_xarray_roundtrips.py :

• test_roundtrip_ixi[ds__dtype__string]
• test_roundtrip_ixi[ds__stringvar__singlepoint]
• test_roundtrip_ixi[ds__stringvar__multipoint]

This is ~ as expected,

• the dtype__string one was disabled before
• the other 2 have been failing since xarray>v2026.02, e.g. here
• however the Python 3.10 failures are different, as this has earlier versions :
  • Python 3.10 --> iris=3.11.1, xarray=2025.6.1 : xarray gives different string data handling
  • cf. Python 3.11 --> iris=3.14.1, xarray=2026.07
  • cf. Python 3.12/3.13 --> iris=3.15.0, xarray=2026.07

[pp-mo]

More changes

• go back to testing against current Iris (should pass tests when 3.16 is released)
• extend support and testing to Python 3.14

[pp-mo]

Expectations:

• Pythons 3.11-3.14 should pass all tests when Iris 3.16.1 appears
• Python 3.10 will still be failing a few things (as at present), due to (same) old versions of iris + xarray
  • our position: we do still support Python 3.10 (at least until October 2026!) , but some tests fail (some results are different)