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

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• Do the proposed changes actually accomplish desired goals?
• Do the proposed changes follow the Astropy coding guidelines?
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• Did the CI pass? If no, are the failures related? If you need to run daily and weekly cron jobs as part of the PR, please apply the "Extra CI" label. Codestyle issues can be fixed by the bot.
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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.