Dropped support for Python 2

[claudep]

[coveralls]

Coverage increased (+0.7%) to 90.061% when pulling 1633cc3d5b3adff41c35214fa5a2af9060e36293 on claudep:droppy2 into 0badc75904cb6efd126f1f01b74ca40af9e92ab8 on emcconville:master.

[claudep]

Note I didn't touch changelog.rst yet.

[emcconville]

Hey thanks for this body of work! I originally started thinking about dropping Python-2 for Wand-0.6.x, but got a lot of push back from legacy users. I think the current Python-2 users make up about 20% of Wand downloads.

[claudep]

Sure, you choose when the best moment is to drop Python 2. I took the risk to prepare the patch without asking first, so don't feel any pressure to push it now. Once done, legacy users can still use wand as long as they pin it in their requirements (e.g. wand < 0.7.x).

[emcconville]

Sounds good to me!