[TST] Upcoming dependency test failures

[github-actions]

The weekly build with nightly wheels from numpy and pandas
has failed. Check the logs for any updates that need to be
made in matplotlib.
https://github.com/matplotlib/matplotlib/actions/runs/3860728474

[ksunden]

Numpy has dropped CPython 3.8 for 1.25+

However, they are still testing on 3.8 for pypy but not cpython so they have dropped building wheels for cpython but the sdist still indicates 3.8 is okay, so the nightly pulls the sdist and tries to build it without the build deps installed.

[ksunden]

Perhaps on the nightly we should add --only-binary or --prefer-binary since we do not have the build deps installed anyway.

[greglucas]

That seems quite reasonable for these future tests since our point isn't to build other packages' wheels for them.

[tacaswell]

We should drop py38 from our main branch test matrix. The NEP29 support window of py38 end April 14, 2023 and mpl 3.8 will likely be June or July 2023.