Python2.7中升级安装pip失败的解决方法：（亲测有效）

错误内容为:

Exception:
Traceback (most recent call last):
  File "D:\Python27\lib\site-packages\pip\basecommand.py", line 211, in main
    status = self.run(options, args)
  File "D:\Python27\lib\site-packages\pip\commands\install.py", line 294, in run
    requirement_set.prepare_files(finder)
  File "D:\Python27\lib\site-packages\pip\req\req_set.py", line 334, in prepare_files
    functools.partial(self._prepare_file, finder))
  File "D:\Python27\lib\site-packages\pip\req\req_set.py", line 321, in _walk_req_to_install
    more_reqs = handler(req_to_install)
  File "D:\Python27\lib\site-packages\pip\req\req_set.py", line 491, in _prepare_file
    session=self.session)
  File "D:\Python27\lib\site-packages\pip\download.py", line 825, in unpack_url
    session,
  File "D:\Python27\lib\site-packages\pip\download.py", line 673, in unpack_http_url
    from_path, content_type = _download_http_url(link, session, temp_dir)
  File "D:\Python27\lib\site-packages\pip\download.py", line 857, in _download_http_url
    stream=True,
  File "D:\Python27\lib\site-packages\pip\_vendor\requests\sessions.py", line 477, in get
    return self.request('GET', url, **kwargs)
  File "D:\Python27\lib\site-packages\pip\download.py", line 373, in request
    return super(PipSession, self).request(method, url, *args, **kwargs)
  File "D:\Python27\lib\site-packages\pip\_vendor\requests\sessions.py", line 465, in request
    resp = self.send(prep, **send_kwargs)
  File "D:\Python27\lib\site-packages\pip\_vendor\requests\sessions.py", line 573, in send
    r = adapter.send(request, **kwargs)
  File "D:\Python27\lib\site-packages\pip\_vendor\cachecontrol\adapter.py", line 46, in send
    resp = super(CacheControlAdapter, self).send(request, **kw)
  File "D:\Python27\lib\site-packages\pip\_vendor\requests\adapters.py", line 419, in send
    raise ConnectTimeout(e, request=request)Exception:
Traceback (most recent call