CI: `isort` GitHub action doesn't report expected issues

[elliot-100]

PR #107 passed all CI checks including isort linting: https://github.com/Olen/Spond/actions/runs/9096951441

Expected behaviour: flag unsorted imports, fail all builds and thus all required checks.

If I run isort on it locally, I get a number of issues:

(spond-LnXcNOlD-py3.8) ~\code\elliot-100_Spond git:[feat/club]
isort . --check-only
ERROR: C:\Users\bb\code\elliot-100_Spond\manual_test_functions.py Imports are incorrectly sorted and/or formatted.
ERROR: C:\Users\bb\code\elliot-100_Spond\transactions.py Imports are incorrectly sorted and/or formatted.
ERROR: C:\Users\bb\code\elliot-100_Spond\spond\club.py Imports are incorrectly sorted and/or formatted.
ERROR: C:\Users\bb\code\elliot-100_Spond\spond\spond.py Imports are incorrectly sorted and/or formatted.
ERROR: C:\Users\bb\code\elliot-100_Spond\tests\test_spond.py Imports are incorrectly sorted and/or formatted.
Skipped 2 files

Should investigate and verify other checks are working OK.

[elliot-100]

Looks like I misconfigured isort in GitHub actions. Will submit PR to fix.

[elliot-100]

Partially fixed by #116.

• For files in spond and tests, CI isort run should now flag unsorted imports using black compatible profile, report the diffs, (which is key to troubleshooting), fail all builds and thus all required checks.
• It should also match behaviour of isort . run locally for files in these folders.

However CI isort run does not look at files outside these folders (it's not clear whether it did before as config was broken), as I could not seem to get the same results as running isort . locally after several attempts - which would make it pretty difficult to write compatible code.

This may not be feasible to fully fix. I think the time is better spent on moving to ruff (https://docs.astral.sh/ruff/) which can replace isort and black (and is compatible with some caveats), and is well documented and supported.

[elliot-100]

To get CI isort to run OK on files outside spond and tests, would need to deal with 'missing imports' issues for example scripts, e.g. ics package not specified in requirements. See very old #79