Declare support for Python 2.7, 3.5, 3.6, and 3.7

The main goal of this commit is to clarify for the PyPI users
which versions of Python are supported.

These changes also:

 - Enable testing for 3.8-dev

 - Replace 3.7-dev with 3.7 in .travis.yml

 - Make Travis CI use xenial
   https://docs.travis-ci.com/user/reference/xenial/

 - Remove obsolete sudo: false
   https://blog.travis-ci.com/2018-11-19-required-linux-infrastructure-migration

Author: Jamim

.travis.yml

@@ -1,11 +1,12 @@
+dist: xenial
 language: python
 python:
     - "nightly"
-    - "3.7-dev"
-    - 3.6
-    - 3.5
-    - 2.7
-sudo: false
+    - "3.8-dev"
+    - "3.7"
+    - "3.6"
+    - "3.5"
+    - "2.7"
 install:
   - pip install --upgrade setuptools pip
   - pip install --upgrade --upgrade-strategy eager --pre -e .[test] pytest-cov codecov 'coverage<5'

setup.py

@@ -84,7 +84,11 @@ def run(self):
         'Operating System :: OS Independent',
         'Programming Language :: Python',
         'Programming Language :: Python :: 2',
+        'Programming Language :: Python :: 2.7',
         'Programming Language :: Python :: 3',
+        'Programming Language :: Python :: 3.5',
+        'Programming Language :: Python :: 3.6',
+        'Programming Language :: Python :: 3.7',
     ],
     install_requires = [
         'traitlets',