ci: Drop tox-venv

Its README says it is mostly obsolete due to improvements in
virtualenv. Using it appears to cause problems related to
pypa/setuptools#1934 because virtualenv
installs the wheel package by default but venv doesn't.

Author: bdarnell

.travis.yml

@@ -39,7 +39,7 @@ jobs:
       env: TOX_ENV=lint
 
 install:
-    - travis_retry pip install tox tox-venv
+    - travis_retry pip install tox
     - python -VV
     - curl-config --version; pip freeze