Extra (unnecessary) 3.12 test run in CI

[A5rocks]

An extra 3.12 Python run happens here:

flake8-async/.github/workflows/ci.yml

Line 47 in 61ee52b

run: python -m tox -e "" -- --no-cov

CI logs where this happens:

https://github.com/python-trio/flake8-async/actions/runs/20048717457/job/57499857248?pr=420#step:6:30

This seemingly adds 2 minutes to CI times...