Remove Python2 compatibility

We've done a lot of work migrating our apps and libs to Python 3, so we
are no longer actively looking to support Python 2. The only known
things left running on Python 2 are a couple of scripts in dm-scripts,
but we have tickets to update these to Python 3 in the immediate future
and they are unlikely to be run before that happens.

.travis.yml

@@ -1,6 +1,5 @@
 language: python
 python:
-  - "2.7"
   - "3.6"
 install:
   - pip install -r requirements-dev.txt