pipインストールエラー：「SSLエラー：[SSL：CERTIFICATE_VERIFY_FAILED]証明書の確認に失敗しました」

pipで何かをしようとすると、次のエラーが発生します。以下の説明に従ってpipをアップグレードするのに疲れましたが、何も起こりません。どんなアイデアがありますか？

Collecting pip
Exception:
Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/pip/basecommand.py", line 215, in main
    status = self.run(options, args)
  File "/usr/lib/python2.7/site-packages/pip/commands/install.py", line 307, in run
    requirement_set.prepare_files(finder)
  File "/usr/lib/python2.7/site-packages/pip/req/req_set.py", line 370, in prepare_files
    ignore_dependencies=self.ignore_dependencies))
  File "/usr/lib/python2.7/site-packages/pip/req/req_set.py", line 587, in _prepare_file
    session=self.session, hashes=hashes)
  File "/usr/lib/python2.7/site-packages/pip/download.py", line 810, in unpack_url
    hashes=hashes
  File "/usr/lib/python2.7/site-packages/pip/download.py", line 649, in unpack_http_url
    hashes)
  File "/usr/lib/python2.7/site-packages/pip/download.py", line 842, in _download_http_url
    stream=True,
  File "/usr/lib/python2.7/site-packages/pip/_vendor/requests/sessions.py", line 487, in get
    return self.request('GET', url, **kwargs)
  File "/usr/lib/python2.7/site-packages/pip/download.py", line 378, in request
    return super(PipSession, self).request(method, url, *args, **kwargs)
  File "/usr/lib/python2.7/site-packages/pip/_vendor/requests/sessions.py", line 475, in request
    resp = self.send(prep, **send_kwargs)
  File "/usr/lib/python2.7/site-packages/pip/_vendor/requests/sessions.py", line 585, in send
    r = adapter.send(request, **kwargs)
  File "/usr/lib/python2.7/site-packages/pip/_vendor/cachecontrol/adapter.py", line 46, in send
    resp = super(CacheControlAdapter, self).send(request, **kw)
  File "/usr/lib/python2.7/site-packages/pip/_vendor/requests/adapters.py", line 477, in send
    raise SSLError(e, request=request)
SSLError: [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed (_ssl.c:618)
You are using pip version 8.1.2, however version 19.0.3 is available.
You should consider upgrading via the 'pip install --upgrade pip' command.