Dev-ops modernisation, roughly simultaneous with 1.4.0 release

[ajjackson]

• Update release tests for new framework #336
• Migrate to tox 4 #332
• Use tox 4 for release testing #333 (merged into 332)
• Split release tests into separate PyPI, Conda-forge #346
  These are generally run at different times and need a different setup.
• Get new conda-forge release tests passing #347
• Create a release Workflow that integrates version-numbering and runs tests before pushing #326
• Experiment with constraints file or uv #345
• Drop macos-13 from run_tests.yml #341
• Only check coverage on ubuntu runs #352

[ajjackson]

The conda-forge release tests are failing on Windows with fairly small deltas on certain eigenvalue calculations. I think this is probably "genuine" as a result of minor Numpy math library differences, but not very severe. For now I'd like to leave them failing and keep an eye on it. From the point of view of this maintenance "epic", the task is complete.