Fix flake8 E231

[MetRonnie]

For some reason, Flake8 was not detecting some problems on python 3.9.

Check List

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• No dependency changes
• Tests are not needed
• No changelog entry needed
• No docs needed
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.

[oliver-sanders]

3.7?

[MetRonnie]

$ mypy --python-version=3.7
[...]/python3.12/site-packages/matplotlib/pyplot.py:246: error: Assignment expressions are only supported in Python 3.8 and greater  [syntax]
Found 1 error in 1 file (errors prevented further checking)

We could split off mypy into its own job on python 3.7. But seeing as we run tests on python 3.7 they should already catch any 3.7-incompatible annotations

[oliver-sanders]

Assignment expressions are only supported in Python 3.8 and greater

I think mypy has reported a legit error here that the tests failed to spot! Thankfully this issue only applies to the developer main loop plugins so is of little consequence. Would introducing an upper pin on matplotlib clear the error?

[MetRonnie]

This version of matplotlib would not install on Python 3.7 so there's no legit error here. It's just that Mypy can't work in Python 3.7 mode when it comes across 3.7-incompatible code in 3rd-party libraries: python/mypy#9972

[MetRonnie]

See also #5736

[MetRonnie]

Wait I just realised that since we moved to linting in its own job on GH Actions, we're not installing matplotlib during that job so this error is not raised anymore!

[oliver-sanders]

Ach dammit, we should probably install the all group, one for another day...

[oliver-sanders]

(re-running the failed test)