box-forced is removed in Matplotlib 3.1

[QuLogic]

When building against Matplotlib 3.1, the docs fail (see log) because box-forced is no longer accepted in Matplotlib.

Problem Description

Docs fail on plot_cartopy_facetgrid.py:

WARNING: /builddir/build/BUILD/xarray-0.12.3/doc/gallery/plot_cartopy_facetgrid.py failed to execute correctly: Traceback (most recent call last):
  File "/usr/lib/python3.7/site-packages/sphinx_gallery/gen_rst.py", line 394, in _memory_usage
    out = func()
  File "/usr/lib/python3.7/site-packages/sphinx_gallery/gen_rst.py", line 382, in __call__
    exec(self.code, self.globals)
  File "/builddir/build/BUILD/xarray-0.12.3/doc/gallery/plot_cartopy_facetgrid.py", line 42, in <module>
    ax.set_aspect('equal', 'box-forced')
  File "/usr/lib/python3.7/site-packages/matplotlib/axes/_base.py", line 1295, in set_aspect
    self.set_adjustable(adjustable, share=share)  # Handle sharing.
  File "/usr/lib/python3.7/site-packages/matplotlib/axes/_base.py", line 1335, in set_adjustable
    cbook._check_in_list(["box", "datalim"], adjustable=adjustable)
  File "/usr/lib/python3.7/site-packages/matplotlib/cbook/__init__.py", line 2164, in _check_in_list
    .format(v, k, ', '.join(map(repr, values))))
ValueError: 'box-forced' is not a valid value for adjustable; supported values are 'box', 'datalim'

Output of xr.show_versions()

I can't run this; it's an automated build. But I can get any relevant versions out of the [root.log](https://kojipkgs.fedoraproject.org/work/tasks/5093/37005093/root.log):

• xarray 0.12.3
• python3-pandas 0.24.1-4.fc31
• python3-numpy 1:1.17.0-2.fc31
• python3-scipy 1.2.1-5.fc31
• python3-netcdf4 1.5.1.2-2.fc31
• python3-zarr 2.3.2-3.fc31
• python3-cftime 1.0.3.4-3.fc31
• python3-rasterio 1.0.24-2.fc31
• python3-Bottleneck 1.2.1-10.fc31
• python3-dask 2.1.0-2.fc31~bootstrap
• python3-matplotlib 3.1.1-1.fc31
• python3-cartopy 0.17.0-6.fc31
• python3-seaborn 0.9.0-6.fc31
• python3-setuptools 41.0.1-4.fc31
• python3-pytest 4.6.4-3.fc31
• python3-ipython 7.6.1-2.fc31
• python3-sphinx 1:2.1.2-2.fc31