Drop support for legacy Python 2

[hugovk]

Rationale

Fix #1186.

Implications

People using Python 2 will not be able to use the most recent release of cartopy.

The Python 2 EOL is coming up on 2020-01-01 and many other projects have pledged to drop support on or before that date: http://python3statement.org

Here's the pip installs for cartopy from PyPI for last month, showing Python 2 use is fairly low:

category	percent	downloads
3.6	43.75%	4,011
2.7	22.87%	2,097
3.5	12.95%	1,187
null	11.19%	1,026
3.7	8.42%	772
3.4	0.69%	63
3.8	0.09%	8
2.6	0.03%	3
3.3	0.01%	1
Total		9,168

Source: pypistats python_minor --last-month cartopy

The addition of python_requires means, with modern pip and setuptools, users will get the last cartopy which did support Python 2.

[pelson]

This is awesome! We can't merge this until after v0.18 is released though, and I'm really concerned that this PR will go very stale in that time. Hopefully you were able to run this through 2to3 or something and it was all autogenerated?

Do you want to keep this open (in the knowledge it is likely to go stale), or shall we close and re-open once we have cut v0.18? The latter is my preference, but I'm open to either approaches.

[hugovk]

Most of the changes (in the second and third commits) were made using a tool (pyupgrade), so it'll be easy to redo this after 0.18 is out.

I'd probably prefer to keep it open so it's visible and not forgotten, but don't have a strong preference. Do whichever fits you feel is best for this project :)

[pelson]

In which case I will go ahead and close this. We have the issue to track the problem, and it links directly to this PR so we can easily enough find this again.

Thanks again for doing this - it was useful to see the kinds of things we are going to be able to remove once py2 is dropped, not to mention all of the features we can start to benefit from.

[QuLogic]

Hey, so I forgot about this PR and put in #1516 and #1517, but they do not cover everything that's here. If you want to re-run the tools and put together a new PR, feel free to do so.

[hugovk]

Heh, will do, thanks!

[hugovk]

Please see PR #1540.