CI: Change minimum dependency build to not use extras
CI: Change minimum dependency build to not use extras

This is motivated by the latest Cartopy (0.20) needing NumPy 1.18, which
goes beyond our current minimum (1.17) and would cut our support window
shorter. Given that we don't really advertise a minimum on the extras,
it's probably best just to make the minium build a no-extras build.

Unidata/MetPy,Unidata/MetPy