pip安装失败“找不到满足要求的版本”

Question

我正在尝试使用pip安装一些软件包，但始终出现以下错误消息。

 pip3 install flask
Collecting flask
  Retrying (Retry(total=4, connect=None, read=None, redirect=None, status=None)) after connection broken by 'ProtocolError('Connection aborted.', ConnectionResetError(104, 'Connection reset by peer'))': /simple/flask/
  Retrying (Retry(total=3, connect=None, read=None, redirect=None, status=None)) after connection broken by 'ProtocolError('Connection aborted.', ConnectionResetError(104, 'Connection reset by peer'))': /simple/flask/
  Retrying (Retry(total=2, connect=None, read=None, redirect=None, status=None)) after connection broken by 'ProtocolError('Connection aborted.', ConnectionResetError(104, 'Connection reset by peer'))': /simple/flask/
  Retrying (Retry(total=1, connect=None, read=None, redirect=None, status=None)) after connection broken by 'ProtocolError('Connection aborted.', ConnectionResetError(104, 'Connection reset by peer'))': /simple/flask/
  Retrying (Retry(total=0, connect=None, read=None, redirect=None, status=None)) after connection broken by 'ProtocolError('Connection aborted.', ConnectionResetError(104, 'Connection reset by peer'))': /simple/flask/
  Could not find a version that satisfies the requirement flask (from versions: )
No matching distribution found for flask

我尝试安装多个软件包（numpy，scipy和matplotlib），所有软件包均失败，并显示类似的错误消息。

我尝试在命令中使用include --trusted-host pypi.org --trusted-host files.pythonhosted.org（如this回答），并使用easy_install，但没有一个起作用。

这不是我的防火墙的问题，因为关闭防火墙后它仍然无法工作。

P.S .：我正在使用虚拟环境，但这似乎并不影响结果。而且，当我尝试通过浏览器访问pypi时，大多数情况下都显示无法访问该网站。