Mergeback from main

* Convert Test___eq__ to pytest.

* Convert Test_combine to pytest.

* Convert Test_difference to pytest.

* Review changes.

Merge-forward to FEATURE_split_attrs

* Tests for attribute handling in netcdf load/save.

* Tidy test functions.

* Fix import order exception.

* Add cf-global attributes test.

* Towards more pytest-y implemenation.

* Replace 'create_testcase' with fixture which also handles temporary directory.

* Much tidy; use fixtures to parametrise over multiple attributes.

* Fix warnings; begin data-style attrs tests.

* Tests for data-style attributes.

* Simplify setup fixture + improve docstring.

* No parallel test runner, to avoid error for Python>3.8.

* Fixed for new-style netcdf module.

* Small review changes.

* Rename attributes set 'data-style' as 'local-style'.

* Simplify use of fixtures; clarify docstrings/comments and improve argument names.

* Clarify testing sections for different attribute 'styles'.

* Re-enable parallel testing.

* Sorted params to avoid parallel testing bug - pytest#432.

* Rename test functions to make alpha-order match order in class.

* Split netcdf load/save attribute testing into separate sourcefile.

* Add tests for loaded cube attributes; refactor to share code between Load and Roundtrip tests.

* Add tests for attribute saving.

* Fix method names in comments.

* Clarify source of Conventions attributes.

* Explain the test numbering in TestRoundtrip/TestLoad.

* Remove obsolete test helper method.

* Fix small typo; Fix numbering of testcases in TestSave.

* Implement split cube attributes.

* Test fixes.

* Modify examples for simpler metadata printouts.

* Added tests, small behaviour fixes.

* Simplify copy.

* Fix doctests.

* Skip doctests with non-replicable outputs (from use of sets).

* Tidy test comments, and add extra test.

* Tiny typo.

* Remove redundant redefinition of Cube.attributes.

* Add CubeAttrsDict in module __all__ + improve docs coverage.

* Review changes - small test changes.

* More review changes.

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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* Fix CubeAttrsDict example docstrings.

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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* Odd small fixes.

* Improved docstrings and comments; fix doctests.

* Don't sidestep netcdf4 thread-safety.

* Publicise LimitedAttributeDict, so CubeAttrsDict can refer to it.

* Fix various internal + external links.

* Update lib/iris/cube.py

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* Update lib/iris/cube.py

Co-authored-by: Martin Yeo <40734014+trexfeathers@users.noreply.github.com>

* Update lib/iris/cube.py

Co-authored-by: Martin Yeo <40734014+trexfeathers@users.noreply.github.com>

* Update lib/iris/cube.py

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* Streamline docs.

* Review changes.

---------

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* Distinguish local+global attributes in netcdf loads.

* Small test fixes.

* Small doctest fix.

* Fix attribute load-save tests for new behaviour, and old-behaviour equivalence.

* Clarification in CubeAttrsDict examples.

* CubeAttrsDict fix docstring typo.

* Raise awareness of split attributes in user guide.

* What's New entry.

* Changes to metadata documentation.

* Add docs and future switch, no function yet.

* Typing enables code completion for Cube.attributes.

* Make roundtrip checking more precise + improve some tests accordingly (cf. #5403).

* Rework all tests to use common setup + results-checking code.

* Saver supports split-attributes saving (no tests yet).

* Tiny docs fix.

* Explain test routines better.

* Fix init of FUTURE object.

* Remove spurious re-test of FUTURE.save_split_attrs.

* Don't create Cube attrs of 'None' (n.b. but no effect as currently used).

* Remove/repair refs to obsolete routines.

* Check all warnings from save operations.

* Remove TestSave test numbers.

* More save cases: no match with missing, and different cube attribute types.

* Run save/roundtrip tests both with+without split saves.

* Fix.

* Review changes.

* Fix changed warning messages.

* Move warnings checking from 'run' to 'check' phase.

* Simplify and improve warnings checking code.

* Fix wrong testcase.

* Minor review changes.

* Fix reverted code.

* Use sets to simplify demoted-attributes code.

* WIP

* Working with iris 3.6.1, no errors TestSave or TestRoundtrip.

* Interim save (incomplete?).

* Different results form for split tests; working for roundtrip.

* Check that all param lists are sorted.

* Check matrix result-files compatibility; add test_save_matrix.

* test_load_matrix added; two types of load result.

* Finalise special-case attributes.

* Small docs tweaks.

* Add some more testcases,

* Ensure valid sort-order for globals of possibly different types.

* Initialise matrix results with legacy values from v3.6.1 -- all matching.

* Add full current matrix results, i.e. snapshot current behaviours.

* Review changes : rename some matrix testcases, for clarity.

* Define common-metadata operartions on split attribute dictionaries.

* Tests for split-attributes handling in CubeMetadata operations.

* Small tidy and clarify.

* Common metadata ops support mixed split/unsplit attribute dicts.

* Clarify with better naming, comments, docstrings.

* Remove split-attrs handling to own sourcefile, and implement as a decorator.

* Remove redundant tests duplicated by matrix testcases.

* Newstyle split-attrs matrix testing, with fewer testcases.

* Small improvements to comments + docstrings.

* Fix logic for equals expectation; expand primary/secondary independence test.

* Clarify result testing in metadata operations decorator.

* Add tests in advance for split-attributes handling cases.

* Move dict conversion inside utility, for use elsewhere.

* Add support for split-attributes to equalise_attributes.

* Update lib/iris/util.py

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* Update lib/iris/tests/unit/util/test_equalise_attributes.py

Co-authored-by: Martin Yeo <40734014+trexfeathers@users.noreply.github.com>

* Simplify and clarify equalise_attributes code.

---------

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* Extra CubeAttrsDict methods to emulate dictionary behaviours.

* Don't use staticmethod on fixture.

* Small improvement to split-attrs whatsnew.

* Emit deprecation warning when saving without split-attrs enabled.

* Stop legacy-split-attribute warnings from upsetting delayed-saving tests.