title: "Cannot fetch index base URL" error message when attempting to install scikit-learn for Python 3
body: <p>I try to install scikit-learn for Python 3. I do it in the following way:</p>
  
  <pre><code>virtualenv model_env
  source  model_env/bin/activate
  pip3 install sklearn
  </code></pre>
  
  <p>As a result I get the following error message:</p>
  
  <pre><code>Downloading/unpacking sklearn
    Cannot fetch index base URL https://pypi.python.org/simple/
    Could not find any downloads that satisfy the requirement sklearn
  Cleaning up...
  No distributions at all found for sklearn
  </code></pre>
  
  <p>I had the same problem with <code>pandas</code> package and I have resolved it by using the following command:</p>
  
  <pre><code>sudo apt-get install python3-pandas
  </code></pre>
  
  <p>Unfortunately, the same approach does not work for the <code>sklearn</code></p>
  
  <pre><code>sudo apt-get install python3-sklearn
  </code></pre>
  
  <p><strong>ADDED</strong></p>
  
  <p>When I replace <code>sklearn</code> by <code>scikit-learn</code>, I have the same problem:</p>
  
  <pre><code>Downloading/unpacking scikit-learn
    Cannot fetch index base URL https://pypi.python.org/simple/
    Could not find any downloads that satisfy the requirement scikit-learn
  Cleaning up...
  No distributions at all found for scikit-learn
  </code></pre>
  
  <p><strong>ADDED 2</strong></p>
  
  <p>As it has been recommended, I have try to use pip in combination with <code>-vvv</code>. Note that I use <code>pip3</code> instead of <code>pip</code>. This is what I get as the result:</p>
  
  <pre><code>Downloading/unpacking scikit-learn
    Getting page https://pypi.python.org/simple/scikit-learn/
    Could not fetch URL https://pypi.python.org/simple/scikit-learn/: connection error: HTTPSConnectionPool(host='pypi.python.org', port=443): Max retries exceeded with url: /simple/scikit-learn/ (Caused by &lt;class 'OSError'&gt;: [Errno 101] Network is unreachable)
    Will skip URL https://pypi.python.org/simple/scikit-learn/ when looking for download links for scikit-learn
    Getting page https://pypi.python.org/simple/
    Could not fetch URL https://pypi.python.org/simple/: connection error: HTTPSConnectionPool(host='pypi.python.org', port=443): Max retries exceeded with url: /simple/ (Caused by &lt;class 'OSError'&gt;: [Errno 101] Network is unreachable)
    Will skip URL https://pypi.python.org/simple/ when looking for download links for scikit-learn
    Cannot fetch index base URL https://pypi.python.org/simple/
    URLs to search for versions for scikit-learn:
    * https://pypi.python.org/simple/scikit-learn/
    Getting page https://pypi.python.org/simple/scikit-learn/
    Could not fetch URL https://pypi.python.org/simple/scikit-learn/: connection error: HTTPSConnectionPool(host='pypi.python.org', port=443): Max retries exceeded with url: /simple/scikit-learn/ (Caused by &lt;class 'OSError'&gt;: [Errno 101] Network is unreachable)
    Will skip URL https://pypi.python.org/simple/scikit-learn/ when looking for download links for scikit-learn
    Could not find any downloads that satisfy the requirement scikit-learn
  Cleaning up...
    Removing temporary dir /tmp/pip_build_root...
  No distributions at all found for scikit-learn
  Exception information:
  Traceback (most recent call last):
    File "/usr/lib/python3/dist-packages/pip/basecommand.py", line 122, in main
      status = self.run(options, args)
    File "/usr/lib/python3/dist-packages/pip/commands/install.py", line 278, in run
      requirement_set.prepare_files(finder, force_root_egg_info=self.bundle, bundle=self.bundle)
    File "/usr/lib/python3/dist-packages/pip/req.py", line 1178, in prepare_files
      url = finder.find_requirement(req_to_install, upgrade=self.upgrade)
    File "/usr/lib/python3/dist-packages/pip/index.py", line 277, in find_requirement
      raise DistributionNotFound('No distributions at all found for %s' % req)
  pip.exceptions.DistributionNotFound: No distributions at all found for scikit-learn
  
  Storing debug log for failure in /home/rngorb/.pip/pip.log
  </code></pre>