gistfile1.txt

$ mkvirtualenv a
New python executable in a/bin/python
Installing setuptools, pip...done.
(a)╭─cyn@ultrabook  ~/desarrollo 
╰─$ pip install --upgrade pip
Downloading/unpacking pip from https://pypi.python.org/packages/3f/ef/935d9296acc4f48d1791ee56a73781271dce9712b059b475d3f5fa78487b/pip-9.0.0-py2.py3-none-any.whl#md5=55f554c8be55cba2a766e40fdb1bb25d
  Downloading pip-9.0.0-py2.py3-none-any.whl (1.3MB): 1.3MB downloaded
Installing collected packages: pip
  Found existing installation: pip 1.5.6
    Uninstalling pip:
      Successfully uninstalled pip
Compiling /home/cyn/.virtualenvs/a/build/pip/pip/_vendor/html5lib/treewalkers/etree.py ...
  File "/home/cyn/.virtualenvs/a/build/pip/pip/_vendor/html5lib/treewalkers/etree.py", line 13
    from pip._vendor. import string_types
                           ^
SyntaxError: invalid syntax

Successfully installed pip