Support matplotlib >= 3.6.0

[riley-brady]

Currently the latest proplot version only supports matplotlib<=3.4.3. Given that matplotlib is up to 3.8.3 and the latest xarray versions require higher versions, we should push to support the latest matplotlib here.

It could be fruitful in the future to run a nightly test suite running the bleeding edge of this repo against the latest matplotlib version to detect this sooner.

The conda feedstock is also locked at matplotlib-base < 3.5.0 due to these issues. See PR here waiting for new proplot release: https://github.com/conda-forge/proplot-feedstock/pull/28.