CI: update pyupgrade to 2.7.4 in pre-commit

[BAlmeidaS]

Updating pyupgrade to 2.7.4 in pre-commit yaml. pre-commit run pyupgrade --all-files runs good 👍

• closes CI upgrade pyupgrade to v2.7.4 #37891
• tests added / passed
• passes black pandas
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry
• passes pre-commit run pyupgrade --all-files

[MarcoGorelli]

Thanks @BAlmeidaS !

@jreback the automated weekly update job from #37362 didn't work: https://github.com/pandas-dev/pandas/actions/runs/365510411

If you added the ACTION_TRIGGER_TOKEN secret token then I don't know why it failed, the logs don't give much away - in which case, I could remove the job and do it manually (as here) when there's an update (which isn't too bad anyway)