Fix matplotlib in upstream-dev test config

[dcherian]

From @keewis comment in #4138

I just noticed that the rackcdn.org repository doesn't have matplotlib>=3.2.0, so since about late February we don't test against matplotlib upstream anymore.

[dcherian]

Maybe @QuLogic or @dopplershift could point us in the right direction?

[tacaswell]

Those wheels are (I think) built from https://github.com/MacPython/matplotlib-wheels/tree/master . We also use that system to build the wheels we push to pypi, but from the build-X.Y branch. The details of how the travis file needed to be configured changed and we never merged those changes back to master (which we have now done). but the builds are failing with an obscure error ( https://travis-ci.org/github/MacPython/matplotlib-wheels/jobs/697078308 ).

[tacaswell]

Thanks to @QuLogic the wheels are building and uploading again.

[keewis]

thanks for the quick fix