CI: upgrade weekly cron devdeps jobs to Python 3.15 (dev) - #20198

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I missed that these jobs were requiring matplotlib, which doesn't have wheels for 3.15 yet. I expect it to happen soon however, so I'm going to keep this PR around as a draft for now.

@neutrinoceros - Matplotlib has Python 3.15 wheels now, maybe we can revive this?

Thanks for the ping, I didn't realize. I just rebased but it won't work out of the box until after matplotlib 3.11.2 is 1 week old, about 15 hours from now

matplotlib is go, but we're still blocked by pyyaml

• Python 3.15 support yaml/pyyaml#951