Do test with py37

.travis.yml

@@ -24,8 +24,12 @@ matrix:
       env:
         - TOXENV=py36
         - PYTEST_ADDOPTS="--cov ./ --cov-append --cov-config setup.cfg"
-    - python: 'nightly'
+    - python: '3.7'
       env: TOXENV=py37
+      dist: xenial
+      sudo: true
+    - python: 'nightly'
+      env: TOXENV=py38
     - python: '3.6'
       env: TOXENV=docs
     - python: '3.6'

tox.ini

@@ -1,13 +1,13 @@
 [tox]
 minversion = 2.0
-envlist = docs,flake8,mypy,coverage,py{27,34,35,36,py},du{11,12,13,14}
+envlist = docs,flake8,mypy,coverage,py{27,34,35,36,37,38,py},du{11,12,13,14}
 
 [testenv]
 usedevelop = True
 passenv =
     https_proxy http_proxy no_proxy PERL PERL5LIB PYTEST_ADDOPTS EPUBCHECK_PATH
 description =
-    py{27,34,35,36,py}: Run unit tests against {envname}.
+    py{27,34,35,36,37,38,py}: Run unit tests against {envname}.
     du{11,12,13,14}: Run unit tests with the given version of docutils.
 
 # TODO(stephenfin) Replace this with the 'extras' config option when tox 2.4 is