ci: fix Travis (#236) (#6786)

Author: blueyed

.travis.yml

@@ -20,7 +20,7 @@ jobs:
     - env: TOXENV=py35 PYTEST_COVERAGE=1 PYTEST_ADDOPTS="-k test_raises_cyclic_reference"
       before_install:
         # Work around https://github.com/jaraco/zipp/issues/40.
-        - python -m pip install -U pip 'setuptools>=34.4.0' virtualenv
+        - python -m pip install -U 'setuptools>=34.4.0' virtualenv==16.7.9
 
 before_script:
   - |