Add pypy3 to tox.ini.

tox.ini

@@ -18,6 +18,7 @@ envlist =
         py33, py33-full,
         py34, py34-full,
         pypy, pypy-full,
+        pypy3,
 
         # Also run the tests with each possible replacement of a default
         # component.  Run each test on both python 2 and 3 where possible.
@@ -142,6 +143,9 @@ deps =
      futures
      mock
 
+[testenv:pypy3]
+basepython = pypy3
+
 [testenv:py2-curl]
 # Same as py27-full, but runs the tests with curl_httpclient by default.
 # Note that httpclient_test is always run with both client implementations;