Drop support for Python 2

[jhamman]

RVIC has been getting more bug report of weird rounding behavior. This is a common issue that arrises when supporting Python 2 and 3 simultaneously. Good news is, core dependencies like numpy have identified a timeline for sunsetting Python 2 (ref: http://www.python3statement.org/). I suggest removing Python 2 support in the next major release. This will help eliminate some of these pesky little bugs.

cc @bartnijssen