All PRs are failing because AppVeyor is reporting a syntax error
AppVeyor tests are all failing with a syntax error:

```python
Using c:\python27_64\envs\_build\lib\site-packages
Finished processing dependencies for pandas==0.19.0+69.g270600c
Compiling C:\Python27_64\envs\_build\Lib\site-packages\pip\_vendor\html5lib\treewalkers\etree.py ...
  File "C:\Python27_64\envs\_build\Lib\site-packages\pip\_vendor\html5lib\treewalkers\etree.py", line 13
    from pip._vendor. import string_types
                           ^
SyntaxError: invalid syntax
```
