Travis: test with Python 3.7

Uses the new Ubuntu Xenial containers, keeping Trusty for pypy.

It might be available there somehow, but currently there is no public list
of on-demand Python versions (travis-ci/travis-ci#8782).

Closes #356.

.scrutinizer.yml

@@ -131,4 +131,4 @@ filter:
 tools:
   external_code_coverage:
     timeout: 1200
-    runs: 5
+    runs: 6

.travis.yml

@@ -1,4 +1,4 @@
-sudo: false
+dist: xenial
 language: python
 env:
   global:
@@ -9,14 +9,17 @@ matrix:
   include:
     - python: 3.6
       env: CI_TARGET=checkqa TOXENV=checkqa
+    - python: pypy
+      dist: trusty
+      sudo: false
 python:
   # If the build matrix gets bigger, also update the number of runs
   # at the bottom of .scrutinizer.yml.
   - 2.7
   - 3.4
   - 3.5
   - 3.6
-  - pypy
+  - 3.7
 before_install:
   - if [ $CI_TARGET = tests ]; then
       eval "$(curl -Ss https://raw.githubusercontent.com/neovim/bot-ci/master/scripts/travis-setup.sh) nightly-x64";