Dropping support for Python 2.7

[dopplershift]

So given that the ecosystem is dropping support for 2.7 in earnest, the time has come for us to start planning for Siphon to move on from Legacy Python:

• Core Python developers will stop support for Python 2.7 January 1, 2020
• NumPy feature releases will be Python 3 only starting January 1, 2019, and support for the last release supporting Python 2 will end January 1, 2020.
• XArray will drop 2.7 January 1, 2019 as well
• Matplotlib's 3.0 release, tentatively Summer 2018, will be Python 3 only; the current 2.2 release will be the last long term release that supports 2.7.

Given these events, and based on preliminary input from Unidata's Users' committee, I propose that Siphon drop support for Python 2.7 in the Fall of 2019. That implies that the Fall 2019 release of Siphon will be the first that supports only Python 3. The release before that (Spring or Summer 2019) will be the last that supports Python 2.7; given Siphon's relatively nascent status, I also propose that this release NOT have any particular long term support status.

I imagine this will impact some of our users, so I welcome feedback here. Note that we don't have the capacity to maintain all of Siphon's upstream dependencies, so some of this transition is beyond our control.