Fix Travis build on Python 3.2

.travis.yml

@@ -28,7 +28,8 @@ matrix:
           env: TOXENV=pypy3
 
 install:
-    - travis_retry pip install tox
+    # "virtualenv<14.0.0" is needed for Python 3.2 compat
+    - travis_retry pip install "virtualenv<14.0.0" tox
 
 script:
     - travis_retry tox