TST: Install mpldev the same way as devdeps for matplotlib - #15290

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Phew we're getting a bit of a break with the ✔️ on devdeps while RCs are out... 😆

Numpy: 1.26.0rc1
Scipy: 1.12.0.dev0+1648.5495f4c
Matplotlib: 3.8.0rc1

But really this PR only affects the CircleCI job with mpldev: https://app.circleci.com/pipelines/github/astropy/astropy/17229/workflows/584c8eea-2b0d-4b72-86d2-18a2ca03d1bd/jobs/114841

Seems to pick up Matplotlib: 3.8.0rc1 that is consistent with devdeps.

I think we good here.