Travis CI: move to Unbutu xenial for all Pythons

Except pypy/pypy3 - keep those on trusty due to https://travis-ci.community/t/pypy-2-7-on-xenial/889 .

.travis.yml

@@ -1,14 +1,27 @@
-sudo: false
 language: python
+dist: xenial
+# require sudo to avoid Travis falling back to trusty; see
+# https://github.com/travis-ci/travis-ci/issues/10110 and also
+# https://github.com/travis-ci/travis-ci/issues/9069
+sudo: required
 
 python:
   - "2.7"
   - "3.4"
   - "3.5"
   - "3.6"
   - "3.7"
-  - "pypy"
-  - "pypy3"
+
+# also test PyPy; do this on Trusty due to
+# https://travis-ci.community/t/pypy-2-7-on-xenial/889
+matrix:
+  include:
+    - python: "pypy"
+      dist: trusty
+      sudo: False
+    - python: "pypy3"
+      dist: trusty
+      sudo: False
 
 install:
   - git config --global user.email "bumpversion-test-git@travis.ci"
@@ -20,4 +33,3 @@ install:
   - pip install tox-travis
 
 script: tox
-