Add Cartopy to Python 3 Statement

[CAM-Gerlach]

Description

As a corollary to #1186 , assuming Cartopy intends to stop supporting Python 2 by the January 1, 2020 deadline (as that issue seems to imply), as an important package used in the atmospheric science community, I suggest adding the project to the Python3Statement, like most of the PyData stack and MetPy have done, to make it easy and clear for users and downstream projects to find in one central place and help accelerate Python 3 adoption across the userbase.

Let me know if you'd like any help or have any questions. Thanks!

[CAM-Gerlach]

@pelson @QuLogic I've opened a draft PR in python3statement/python3statement.github.io#215 to illustrate; of course, it will not be merged without your explicit approval. Just let us know over there whether it looks good to you and if you want us to go ahead with it. Thanks!

[dopplershift]

I don't see any reason not to go ahead with it since we've established we're dropping Python 2 by that date.

[QuLogic]

This was done some time back.