add pypy3 as a supported environment, by default

.travis.yml

@@ -18,6 +18,12 @@ matrix:
     # core
     - python: "3.6"
       env: TOX_POSARGS="-e py36-core"
+    #
+    # pypy3 testing
+    #
+    # core
+    - python: "pypy3"
+      env: TOX_POSARGS="-e pypy3-core"
 cache:
   - pip: true
 install:

setup.py

@@ -62,5 +62,6 @@ setup(
         'Programming Language :: Python :: 3',
         'Programming Language :: Python :: 3.5',
         'Programming Language :: Python :: 3.6',
+        'Programming Language :: Python :: Implementation :: PyPy3',
     ],
 )

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 envlist=
-    py{35,36}-core
+    py{35,36,py3}-core
     lint
 
 [isort]
@@ -25,6 +25,7 @@ commands=
 basepython =
     py35: python3.5
     py36: python3.6
+    pypy3: pypy3
 extras=test
 
 [testenv:lint]