title: "pip install json" fails on Ubuntu
body: <p>Cannot install the json module.
  As far as I know I shouldn't use sudo. what's the matter?</p>
  
  <pre><code> pip install json
  The directory '/home/snow/.cache/pip/http' or its parent directory is not owned by the current user and the cache has been disabled. Please check the permissions and owner of that directory. If executing pip with sudo, you may want sudo's -H flag.
  The directory '/home/snow/.cache/pip' or its parent directory is not owned by the current user and caching wheels has been disabled. check the permissions and owner of that directory. If executing pip with sudo, you may want sudo's -H flag.
  Collecting json
    Downloading json-99.0.tar.gz
      Complete output from command python setup.py egg_info:
      Traceback (most recent call last):
        File "&lt;string&gt;", line 1, in &lt;module&gt;
        File "/tmp/pip-build-YkvqVh/json/setup.py", line 2, in &lt;module&gt;
          raise RuntimeError("Package 'json' must not be downloaded from pypi")
      RuntimeError: Package 'json' must not be downloaded from pypi
  
      ----------------------------------------
  Command "python setup.py egg_info" failed with error code 1 in /tmp/pip-build-YkvqVh/json/
  </code></pre>