Improve CI

- Bump CI to use Python 3.7 (and test against 3.7)

Python 3.7 has been out for a while now, seems sensible to test
against it and upgrade to it.

- Remove the sudo part

It has been deprecated
https://blog.travis-ci.com/2018-11-19-required-linux-infrastructure-migration.

- Be specific about the distribution

This is required for 3.7 (not available in trusty) and is for clarity
elsewhere.

Author: pgjones

.travis.yml

@@ -1,4 +1,3 @@
-sudo: false
 language: python
 
 branches:
@@ -15,28 +14,39 @@ matrix:
   include:
     # Main tests on supported Python versions.
     - python: "2.7"
+      dist: trusty
       env: TOXENV=py27
     - python: "3.4"
+      dist: trusty
       env: TOXENV=py34
     - python: "3.5"
+      dist: trusty
       env: TOXENV=py35
     - python: "3.6"
+      dist: trusty
       env: TOXENV=py36
+    - python: "3.7"
+      dist: xenial
+      env: TOXENV=py37
     - python: "pypy-5.3.1"
+      dist: trusty
       env: TOXENV=pypy
 
     # Linting, docs, and other non-test stuff.
-    - python: "3.4"
+    - python: "3.7"
+      dist: xenial
       env: TOXENV=lint
-    - python: "3.5"
+    - python: "3.7"
+      dist: xenial
       env: TOXENV=docs
-    - python: "2.7"
+    - python: "3.7"
+      dist: xenial
       env: TOXENV=packaging
 
     # Test we haven't broken our major dependencies.
     - python: "2.7"
+      dist: trusty
       env: TOXENV=py27-twistedMaster
-
 install:
   - "pip install -U pip setuptools"
   - "pip install -U tox"

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py27, py34, py35, py36, pypy, lint, packaging, docs
+envlist = py27, py34, py35, py36, py37, pypy, lint, packaging, docs
 
 [testenv]
 deps= -r{toxinidir}/test_requirements.txt
@@ -25,12 +25,12 @@ deps =
 commands = python -m twisted.trial --reporter=text twisted
 
 [testenv:lint]
-basepython=python3.4
+basepython=python3.7
 deps = flake8==3.5.0
 commands = flake8 --max-complexity 10 h2 test
 
 [testenv:docs]
-basepython=python3.5
+basepython=python3.7
 deps = sphinx==1.4.9
 changedir = {toxinidir}/docs
 whitelist_externals = rm
@@ -45,7 +45,7 @@ commands =
     python visualizer/visualize.py -i docs/source/_static
 
 [testenv:packaging]
-basepython=python2.7
+basepython=python3.7
 deps =
     check-manifest==0.36
     readme-renderer==17.3