Test running pyt in a python 3.7 env

It should just work. There were no AST changes.

Also removes the unnecesary whitelist_external from tox.ini.

Note: pyt can be run in a py36 environment on code targeting 2.7-3.8.
There is no requirement for pyt to be run under the same environment as
your code is intended to run in.

Running pyt under 3.8 will require some further work due to the change
from ast.Str to ast.Constant:
https://github.com/python/cpython/pull/9445/files
https://docs.python.org/dev/whatsnew/3.8.html#deprecated
https://bugs.python.org/issue32892

Author: bcaller

.travis.yml

@@ -1,6 +1,8 @@
 language: python
+dist: xenial
 python:
   - "3.6"
+  - "3.7"
 install:
   - pip install codeclimate-test-reporter 'coverage>=4.0,<4.4' flake8
 before_script:

tox.ini

@@ -1,12 +1,11 @@
 [tox]
-envlist = py36,cover,lint
+envlist = py36,py37,cover,lint
 
 [testenv]
 commands =
     python -m tests
 
 [testenv:cover]
-whitelist_externals = coverage
 deps =
     coverage>=4.0,<4.4
 commands =