Update pcolormesh with new matplotlib versions

[greglucas]

Description

The current master of matplotlib has changed the keyword arguments for pcolor*. See: matplotlib/matplotlib#16258

We will need to change the _pcolormesh_patched function. Specifically, the call to _pcolor_args and the shading option and get rid of allmatch when that update from mpl is released.

Note: I didn't find any version checks and if statements to handle cases like this for MPL_VERSION in the code, only in the tests, so I'm also not sure how we want to handle this.

[dopplershift]

Just to add that this is slated for release with matplotlib 3.3.