MNT: Update settings for building matplotlib on CI.
MNT: Update settings for building matplotlib on CI.

This should help test results match the "normal" wheels from upstream.

[skip appveyor]
ahaberlie/MetPy,jrleeman/MetPy,dopplershift/MetPy,ahill818/MetPy,ShawnMurd/MetPy,Unidata/MetPy,ahaberlie/MetPy,Unidata/MetPy,dopplershift/MetPy,jrleeman/MetPy