Mergeback v3.0.x branch into main

[pp-mo]

Needs doing before we create a v3.1.x branch.
Unfortunately, checking seems to show that will not be trivial, as various CI and infrastructure changes have been implemented in parallel in 'main' and 'v3.0.x' and so there is now quite a lot to resolve (nox, cirrus, isort etc).

[pp-mo]

various CI and infrastructure changes have been implemented in parallel

Just as note of what i mean...
Output following a simple merge of v3.0.x into main :

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	both added:      .cirrus.yml
	both modified:   README.md
	deleted by us:   docs/iris/gallery_code/meteorology/plot_COP_maps.py
	deleted by us:   docs/iris/src/common_links.inc
	deleted by us:   docs/iris/src/developers_guide/ci_checks.png
	deleted by us:   docs/iris/src/developers_guide/contributing_ci_tests.rst
	deleted by us:   docs/iris/src/developers_guide/release.rst
	deleted by us:   docs/iris/src/techpapers/change_management.rst
	deleted by us:   docs/iris/src/whatsnew/3.0.rst
	both modified:   docs/src/developers_guide/contributing_code_formatting.rst
	both modified:   docs/src/developers_guide/contributing_graphics_tests.rst
	both modified:   docs/src/developers_guide/contributing_running_tests.rst
	both modified:   docs/src/developers_guide/gitwash/development_workflow.rst
	both modified:   docs/src/installing.rst
	added by them:   docs/src/whatsnew/3.0.1.rst
	added by them:   docs/src/whatsnew/3.0.2.rst
	added by them:   docs/src/whatsnew/3.0.3.rst
	added by them:   docs/src/whatsnew/3.0.4.rst
	both modified:   docs/src/whatsnew/index.rst
	both modified:   lib/iris/common/resolve.py
	both modified:   lib/iris/coord_categorisation.py
	deleted by us:   lib/iris/fileformats/_pyke_rules/fc_rules_cf.krb
	both modified:   lib/iris/fileformats/netcdf.py
	both modified:   lib/iris/fileformats/pp.py
	both added:      lib/iris/tests/unit/common/resolve/test_Resolve.py
	both modified:   lib/iris/tests/unit/fileformats/pp/test_PPDataProxy.py
	both modified:   lib/iris/tests/unit/fileformats/pp_load_rules/test__convert_time_coords.py
	both added:      noxfile.py
	both added:      requirements/ci/nox.lock/py37-linux-64.lock
	both added:      requirements/ci/nox.lock/py38-linux-64.lock
	both modified:   requirements/ci/py37.yml
	both modified:   requirements/ci/py38.yml
	deleted by us:   requirements/core.txt
	deleted by us:   requirements/setup.txt
	deleted by them: setup.cfg
	both modified:   setup.py

I'm not really clear why files which were deleted or added by one 'side' would be included in this list.
This can probably be much improved by merging individual commits from 3.0.x instead.

[rcomer]

Some that are mark “deleted by us” are actually still there but have been both moved and modified in main. E.g. the extra iris subdirectory was nuked from docs and I updated the plot_COP_maps.py example.

[bjlittle]

As per this v3.0.x discussion comment, this is a task on me, which I'm now looking at 👍