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.

.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
 

tox.ini

@@ -11,9 +11,6 @@
 # 'port install curl +universal' to get both 32- and 64-bit versions of
 # libcurl.
 [tox]
-# When tox is run from a venv (instead of a virtualenv), it can get confused
-# unless tox-venv is also installed.
-requires = tox-venv
 envlist =
         # Basic configurations: Run the tests for each python version.
         py35-full,py36-full,py37-full,py38-full,pypy3-full