CU-86948uv4g docstring signature consistency

[mart-r]

I've just realised that my previous PR on the same task was a flop.

The reason flake8 was checking doc strings and comparing them with the method signatures was because I had darglint installed locally. Not because of the newer flake8 version as I had assumed.

I hadn't realised that darglint would silently work with flake8.

So what this PR does is add darglint to dev requirements.
This will force flake8 to run through and check the doc strings match methods signatures on the GHA side just as it did for me locally when I was making the various changes before.

PS:
This is likely to add 2-3 minutes to each GitHub Actions run because darglint is slow.

[tomolopolis]

Task linked: CU-86948uv4g Ensure docstrings match signature

[tomolopolis]

lgtm