Update CI Pythons.

- Add Python 3.8.
- Unpin pypy2.7 and pypy3.

.travis.yml

@@ -1,17 +1,16 @@
 os: linux
-dist: trusty
+dist: xenial
 language: python
 python:
-- 2.6
 - 2.7
-- 3.2
-- 3.3
 - 3.4
 - 3.5
 - 3.6
-- &mainstream_python 3.6
-- pypy2.7-5.8.0
-- pypy3.5-5.8.0
+- 3.7
+- &mainstream_python 3.8
+- nightly
+- pypy
+- pypy3
 install:
 - pip install --upgrade --force-reinstall "setuptools; python_version != '3.2' and python_version != '3.3'" "setuptools < 30; python_version == '3.2'" "setuptools < 40; python_version == '3.3'"
 - pip uninstall --yes six || true
@@ -27,10 +26,12 @@ script:
 jobs:
   fast_finish: true
   include:
-  - python: 3.7
-    dist: xenial  # required for Python >= 3.7 (travis-ci/travis-ci#9069)
-  - python: nightly
-    dist: xenial  # required for Python >= 3.7 (travis-ci/travis-ci#9069)
+  - python: 2.6
+    dist: trusty
+  - python: 3.2
+    dist: trusty
+  - python: 3.3
+    dist: trusty
   - stage: upload new version of python package to PYPI (only for tagged commits)
     python: *mainstream_python
     install: skip

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist=py26,py27,py31,py32,py33,py34,pypy,flake8
+envlist=py26,py27,py32,py33,py34,py35,py36,py37,py38,pypy,flake8
 
 [testenv]
 deps= pytest