CI: re-jigger test matrix to include 3.7

 - run 3.7 with pep8
 - only run 1 3.5 build
 - bump ubuntu version, sudo seems to be required for 3.7

Author: tacaswell

.travis.yml

@@ -1,6 +1,6 @@
 language: python
 
-dist: trusty
+dist: xenial
 sudo: false
 
 branches:
@@ -69,17 +69,17 @@ env:
 matrix:
   include:
     - python: 3.5
+      dist: trusty
       # pytest-cov>=2.3.1 due to https://github.com/pytest-dev/pytest-cov/issues/124.
       env:
         - PINNEDVERS='-c requirements/testing/travis35.txt'
-    - python: 3.5
-      env:
-        # - PYTHONOPTIMIZE=2  # This currently doesn't work.
     - python: 3.6
       env:
         - DELETE_FONT_CACHE=1
         - EXTRAREQS='-r requirements/testing/travis36.txt'
         - RUN_FLAKE8=1
+    - python: 3.7
+      sudo: true
     - python: "nightly"
       env: PRE=--pre
     - os: osx