Progress: installing numpy first has moved the error to pillow needing to be compiled, but not finding the installed copy of zlib.

I think we tookout too much here: https://github.com/matplotlib/matplotlib/pull/15193/files#diff-a3fee1253ef9cd8f9d05eaa692bbe035L28-L29

Looking at pillow's CI, they use appveyor and have a somewhat complicated set of scripts to set up the build enviroment.

It looks like there are already py38 wheels available (https://www.lfd.uci.edu/~gohlke/pythonlibs/#pillow) but I am not sure the best way to get that installed.

The other option is if we don't need to save compressed PNGs we could in theory disable the need for zlib: https://pillow.readthedocs.io/en/latest/installation.html#external-libraries

Perhaps another option is to do the python preview testing on Linux, where hopefully it will be easier to deal with the pillow compilation requirements?

We should run it on both ;)

Hopefully the patch release pre-releases will be easier to manage (as the wheels from the 3.8.0 should still work)

@tacaswell re: chris gohlke's windows wheels: for mplcairo (where I need the cairocffi wheel for the windows build) I just downloaded manually chris' wheel and mirrored it myself https://github.com/anntzer/cairocffi-windows-wheels https://github.com/matplotlib/mplcairo/blob/144388bf94d4cfb8010e835ae5c3e13643533354/azure-pipelines.yml#L53

Not depending on zlib was intentional in #15193 -- I'm effectively relying on Pillow windows wheels being available.

Can we not install zlib at all then?

that's the intent of #15378.

The mac fix went in via #15429 and a hacky way of skipping 38 on azure came in via #15447 .