Add Python 3.8 to tox.ini and setup.cfg (#1492)

Author: jdufresne

setup.cfg

@@ -33,6 +33,7 @@ classifiers =
     Programming Language :: Python :: 3.5
     Programming Language :: Python :: 3.6
     Programming Language :: Python :: 3.7
+    Programming Language :: Python :: 3.8
 
 [options]
 packages = find:

tox.ini

@@ -4,6 +4,7 @@ envlist = py27,
           py35,
           py36,
           py37,
+          py38,
           pypy,
           pypy3,
           coverage,
@@ -86,7 +87,7 @@ commands = coverage combine
            coverage xml -o {toxworkdir}/coverage.xml
            coverage html -d {toxworkdir}/htmlcov
            diff-cover --compare-branch {env:DIFF_AGAINST:origin/master} {toxworkdir}/coverage.xml
-depends = py27, py34, py35, py36, py37, pypy, pypy3
+depends = py27, py34, py35, py36, py37, py38, pypy, pypy3
 parallel_show_output = True
 
 [testenv:exit_code]