Don't install matplotlib by default

[campanelli-sunpower]

matplotlib is SO HEAVY. Let's make it an optional install.

Also, I'm not convinced that future or six are needed anymore, and it would be nice to move from setup.py to pyproject.toml.

My biggest concern is breaking pvlib-python consumption.

[anomam]

100% agree, my dream was to completely decouple the geometry/plotting/resolver layers and allow some deps like maplotlib to be optional, but since I changed jobs last year I've been under the water and I clearly haven't had time for such large endeavors... : (