Clarify plans for future Python 2 support past the EoL, and include on Python3Statement as appropriate

[CAM-Gerlach]

As Cartopy, and many other dependencies (Numpy, Matplotlib, etc) will stop supporting Python 2 on or before the January 1, 2020 deadline, as an important package used in the atmospheric science community, I suggest publicly making clear your plans on whether and when you will drop Python 2 support. If you do plan to do it on or before January 2020, 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]

@pp-mo If you'd like, I can open a draft PR over on python3statement/python3statement.github.io that can be merged if you decide to sign, so you don't actually have to do anything, aside from give us the final approval or not. Lmk, thanks.

[bjlittle]

@CAM-Gerlach Thanks for the nudge... see python3statement/python3statement.github.io#230

[bjlittle]

Closed by python3statement/python3statement.github.io#230

See https://python3statement.org/