title: " TypeError: unsupported operand type(s) for -=: 'Retry' and 'int' " during PIP upgrade
body: <p>I'm using Ubuntu 16.04.2 LTS in an Acer Laptop and I'm trying to update my pip installation (current version: 8.1.1). When I run the command </p>
  
  <p><strong>pip install --upgrade pip</strong></p>
  
  <p>I get the following output:</p>
  
  <pre><code>Exception:
  Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/pip/basecommand.py", line 209, in main
      status = self.run(options, args)
  File "/usr/lib/python2.7/dist-packages/pip/commands/install.py", line 328, in run
      wb.build(autobuilding=True)
  File "/usr/lib/python2.7/dist-packages/pip/wheel.py", line 748, in build
      self.requirement_set.prepare_files(self.finder)
  File "/usr/lib/python2.7/dist-packages/pip/req/req_set.py", line 360, in prepare_files
      ignore_dependencies=self.ignore_dependencies))
  File "/usr/lib/python2.7/dist-packages/pip/req/req_set.py", line 512, in _prepare_file
      finder, self.upgrade, require_hashes)
  File "/usr/lib/python2.7/dist-packages/pip/req/req_install.py", line 273, in populate_link
      self.link = finder.find_requirement(self, upgrade)
  File "/usr/lib/python2.7/dist-packages/pip/index.py", line 442, in find_requirement
      all_candidates = self.find_all_candidates(req.name)
  File "/usr/lib/python2.7/dist-packages/pip/index.py", line 400, in find_all_candidates
      for page in self._get_pages(url_locations, project_name):
  File "/usr/lib/python2.7/dist-packages/pip/index.py", line 545, in _get_pages
      page = self._get_page(location)
  File "/usr/lib/python2.7/dist-packages/pip/index.py", line 648, in _get_page
      return HTMLPage.get_page(link, session=self.session)
  File "/usr/lib/python2.7/dist-packages/pip/index.py", line 757, in get_page
      "Cache-Control": "max-age=600",
  File "/usr/share/python-wheels/requests-2.9.1-py2.py3-none-any.whl/requests/sessions.py", line 480, in get
      return self.request('GET', url, **kwargs)
  File "/usr/lib/python2.7/dist-packages/pip/download.py", line 378, in request
      return super(PipSession, self).request(method, url, *args, **kwargs)
  File "/usr/share/python-wheels/requests-2.9.1-py2.py3-none-any.whl/requests/sessions.py", line 468, in request
      resp = self.send(prep, **send_kwargs)
  File "/usr/share/python-wheels/requests-2.9.1-py2.py3-none-any.whl/requests/sessions.py", line 576, in send
      r = adapter.send(request, **kwargs)
  File "/usr/share/python-wheels/CacheControl-0.11.5-py2.py3-none-any.whl/cachecontrol/adapter.py", line 46, in send
      resp = super(CacheControlAdapter, self).send(request, **kw)
  File "/usr/share/python-wheels/requests-2.9.1-py2.py3-none-any.whl/requests/adapters.py", line 376, in send
      timeout=timeout
  File "/usr/share/python-wheels/urllib3-1.13.1-py2.py3-none-any.whl/urllib3/connectionpool.py", line 610, in urlopen
      _stacktrace=sys.exc_info()[2])
  File "/usr/share/python-wheels/urllib3-1.13.1-py2.py3-none-any.whl/urllib3/util/retry.py", line 228, in increment
      total -= 1
  TypeError: unsupported operand type(s) for -=: 'Retry' and 'int'
  </code></pre>
  
  <p>I have read the messages in the following pages:</p>
  
  <ul>
  <li><a href="https://stackoverflow.com/questions/37495375/python-pip-install-throws-typeerror-unsupported-operand-types-for-retry">Python PIP Install throws TypeError: unsupported operand type(s) for -=: &#39;Retry&#39; and &#39;int&#39;</a></li>
  <li><a href="https://github.com/pypa/pip/issues/3943" rel="nofollow noreferrer">https://github.com/pypa/pip/issues/3943</a></li>
  <li><a href="https://github.com/pypa/pip/issues/4133" rel="nofollow noreferrer">https://github.com/pypa/pip/issues/4133</a></li>
  </ul>
  
  <p>I'm not using any proxy or firewall (unless you count my router as both, idk).</p>