use Python 2.7 to generate documentation

Author: gaborbernat

readthedocs.yml

@@ -1,7 +1,7 @@
 build:
   image: latest
 python:
-  version: 3.6
+  version: 2.7
   pip_install: true
   extra_requirements:
   - docs

src/virtualenv.py

@@ -1,6 +1,6 @@
 #!/usr/bin/env python
 """Create a "virtual" Python installation"""
-
+raise ValueError
 # fmt: off
 import os  # isort:skip
 import sys  # isort:skip