title: pip fails with SSLError on MacOS because of outdated certificate - how to recover?
body: <p>When searching or installing packages with pip on MacOS (10.11.6), I am getting an SSL error similar to the one reproduced below. </p>
  
  <p>The problem occurs for both python2 and python3. I acquired the versions (2.7.14 and 3.6.5) via <a href="https://www.python.org/" rel="nofollow noreferrer">www.python.org</a> as Frameworks. The version of pip was 9.0.1. The problem is new (April 2018) and seems related to an update of OSX.</p>
  
  <p>How to recover from this problem?</p>
  
  <pre><code>$ pip2 search numpy
  
  Exception:
  Traceback (most recent call last):
    File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/pip/basecommand.py", line 215, in main
      status = self.run(options, args)
    File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/pip/commands/search.py", line 45, in run
      pypi_hits = self.search(query, options)
    File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/pip/commands/search.py", line 62, in search
      hits = pypi.search({'name': query, 'summary': query}, 'or')
    File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/xmlrpclib.py", line 1243, in __call__
      return self.__send(self.__name, args)
    File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/xmlrpclib.py", line 1602, in __request
      verbose=self.__verbose
    File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/pip/download.py", line 775, in request
      headers=headers, stream=True)
    File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/pip/_vendor/requests/sessions.py", line 522, in post
      return self.request('POST', url, data=data, json=json, **kwargs)
    File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/pip/download.py", line 386, in request
      return super(PipSession, self).request(method, url, *args, **kwargs)
    File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/pip/_vendor/requests/sessions.py", line 475, in request
      resp = self.send(prep, **send_kwargs)
    File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/pip/_vendor/requests/sessions.py", line 596, in send
      r = adapter.send(request, **kwargs)
    File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/pip/_vendor/cachecontrol/adapter.py", line 47, in send
      resp = super(CacheControlAdapter, self).send(request, **kw)
    File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/pip/_vendor/requests/adapters.py", line 497, in send
      raise SSLError(e, request=request)
  SSLError: [SSL: TLSV1_ALERT_PROTOCOL_VERSION] tlsv1 alert protocol version (_ssl.c:661)
  </code></pre>