CI is failing for Python 3.11

[GraemeWatt]

The nightly CI failed for Python 3.11 on 23rd June 2024 with error messages like Coverage.py warning: No data was collected. (no-data-collected) and SystemError: null argument to internal routine. The same error messages were obtained on re-running the failed CI jobs, but not with other Python versions or when using Python 3.11 locally. I'm not sure of the reason, but the only change I can see is that coverage 7.5.4 was released on 22nd June 2024. There was a pin coverage<6.0 made in commit dbd0e79 , but it looks like the issue prompting the pin was resolved already in coverage 6.0.1. Removing the pin coverage<6.0 allows the CI to pass with Python 3.11.