Recibo este error cada vez que intento instalar cualquier módulo usando pip, pero easy_install funciona perfectamente. No tengo proxies configurados en mi máquina Ubuntu 12.04. Anteriormente funcionaba bien, no sé cómo dejó de funcionar de repente.

Este es el error que obtengo, mientras se ejecutasudo pip install <any_package_name>:

Exception:
Traceback (most recent call last):
  File "/tmp/tmpOA61D3/pip.zip/pip/basecommand.py", line 246, in main
    status = self.run(options, args)
  File "/tmp/tmpOA61D3/pip.zip/pip/commands/install.py", line 342, in run
    requirement_set.prepare_files(finder)
  File "/tmp/tmpOA61D3/pip.zip/pip/req/req_set.py", line 345, in prepare_files
    functools.partial(self._prepare_file, finder))
  File "/tmp/tmpOA61D3/pip.zip/pip/req/req_set.py", line 290, in _walk_req_to_install
    more_reqs = handler(req_to_install)
  File "/tmp/tmpOA61D3/pip.zip/pip/req/req_set.py", line 415, in _prepare_file
    req_to_install, finder)
  File "/tmp/tmpOA61D3/pip.zip/pip/req/req_set.py", line 376, in _check_skip_installed
    finder.find_requirement(req_to_install, self.upgrade)
  File "/tmp/tmpOA61D3/pip.zip/pip/index.py", line 425, in find_requirement
    all_versions = self._find_all_versions(req.name)
  File "/tmp/tmpOA61D3/pip.zip/pip/index.py", line 349, in _find_all_versions
    index_locations = self._get_index_urls_locations(project_name)
  File "/tmp/tmpOA61D3/pip.zip/pip/index.py", line 323, in _get_index_urls_locations
    page = self._get_page(main_index_url)
  File "/tmp/tmpOA61D3/pip.zip/pip/index.py", line 789, in _get_page
    return HTMLPage.get_page(link, session=self.session)
  File "/tmp/tmpOA61D3/pip.zip/pip/index.py", line 878, in get_page
    "Cache-Control": "max-age=600",
  File "/tmp/tmpOA61D3/pip.zip/pip/_vendor/requests/sessions.py", line 476, in get
    return self.request('GET', url, **kwargs)
  File "/tmp/tmpOA61D3/pip.zip/pip/download.py", line 367, in request
    return super(PipSession, self).request(method, url, *args, **kwargs)
  File "/tmp/tmpOA61D3/pip.zip/pip/_vendor/requests/sessions.py", line 464, in request
    resp = self.send(prep, **send_kwargs)
  File "/tmp/tmpOA61D3/pip.zip/pip/_vendor/requests/sessions.py", line 576, in send
    r = adapter.send(request, **kwargs)
  File "/tmp/tmpOA61D3/pip.zip/pip/_vendor/cachecontrol/adapter.py", line 46, in send
    resp = super(CacheControlAdapter, self).send(request, **kw)
  File "/tmp/tmpOA61D3/pip.zip/pip/_vendor/requests/adapters.py", line 370, in send
    timeout=timeout
  File "/tmp/tmpOA61D3/pip.zip/pip/_vendor/requests/packages/urllib3/connectionpool.py", line 544, in urlopen
    body=body, headers=headers)
  File "/tmp/tmpOA61D3/pip.zip/pip/_vendor/requests/packages/urllib3/connectionpool.py", line 341, in _make_request
    self._validate_conn(conn)
  File "/tmp/tmpOA61D3/pip.zip/pip/_vendor/requests/packages/urllib3/connectionpool.py", line 762, in _validate_conn
    conn.connect()
  File "/tmp/tmpOA61D3/pip.zip/pip/_vendor/requests/packages/urllib3/connection.py", line 238, in connect
    ssl_version=resolved_ssl_version)
  File "/tmp/tmpOA61D3/pip.zip/pip/_vendor/requests/packages/urllib3/contrib/pyopenssl.py", line 296, in ssl_wrap_socket
    cnx.set_tlsext_host_name(server_hostname)
AttributeError: '_socketobject' object has no attribute 'set_tlsext_host_name'