title: pip install errors out: SyntaxError: invalid syntax
body: <p>pip install does not work when trying to install virtualenv, requests or pex on CentOS6. I am on python2.6 and pip 9.0.1. Can anyone tell me why is this happening?</p>
  
  <pre><code>(pex_build)[root@pex pex_build]# pip install virtualenv
  </code></pre>
  
  <p>Output:</p>
  
  <pre><code>Traceback (most recent call last):
    File "/opt/pex_build/bin/pip", line 7, in &lt;module&gt;
      from pip._internal import main
    File "/opt/pex_build/lib/python2.6/site-packages/pip/_internal/__init__.py", line 42, in &lt;module&gt;
      from pip._internal import cmdoptions
    File "/opt/pex_build/lib/python2.6/site-packages/pip/_internal/cmdoptions.py", line 16, in &lt;module&gt;
      from pip._internal.index import (
    File "/opt/pex_build/lib/python2.6/site-packages/pip/_internal/index.py", line 526
      {str(c.version) for c in all_candidates},
                        ^
  SyntaxError: invalid syntax
  </code></pre>
  
  <p>Command:</p>
  
  <pre><code>(pex_build) [root@pex pex_build]# pip install requests pex
  </code></pre>
  
  <p>Output: </p>
  
  <pre><code>Traceback (most recent call last):
    File "/opt/pex_build/bin/pip", line 7, in &lt;module&gt;
      from pip._internal import main
    File "/opt/pex_build/lib/python2.6/site-packages/pip/_internal/__init__.py", line 42, in &lt;module&gt;
      from pip._internal import cmdoptions
    File "/opt/pex_build/lib/python2.6/site-packages/pip/_internal/cmdoptions.py", line 16, in &lt;module&gt;
      from pip._internal.index import (
    File "/opt/pex_build/lib/python2.6/site-packages/pip/_internal/index.py", line 526
      {str(c.version) for c in all_candidates},
                        ^
  SyntaxError: invalid syntax
  </code></pre>
  
  <p>Also curl gives the similar error, when trying to get get-pip.py</p>
  
  <p>Command:</p>
  
  <pre><code>(pex_build) [root@pex pex_build]# curl https://bootstrap.pypa.io/get-pip.py | python
  
  
  
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                   Dload  Upload   Total   Spent    Left  Speed
  100 1602k  100 1602k    0     0  7373k      0 --:--:-- --:--:-- --:--:-- 14.6M
  
  Traceback (most recent call last):
    File "&lt;stdin&gt;", line 20636, in &lt;module&gt;
    File "&lt;stdin&gt;", line 197, in main
    File "&lt;stdin&gt;", line 82, in bootstrap
    File "/tmp/tmp5zrn_f/pip.zip/pip/_internal/__init__.py", line 42, in &lt;module&gt;
    File "/tmp/tmp5zrn_f/pip.zip/pip/_internal/cmdoptions.py", line 16, in &lt;module&gt;
    File "/tmp/tmp5zrn_f/pip.zip/pip/_internal/index.py", line 526
      {str(c.version) for c in all_candidates},
                        ^
  SyntaxError: invalid syntax
  </code></pre>