使用pip安装任何包，输出如下：
txs@txs-PC:~$ python3 -m pip install --upgrade pip
WARNING: Retrying (Retry(total=4, connect=None, read=None, redirect=None, status=None)) after connection broken by 'ProxyError('Cannot connect to proxy.', NewConnectionError(': Failed to establish a new connection: [Errno 111] 拒绝连接',))': /simple/pip/
WARNING: Retrying (Retry(total=3, connect=None, read=None, redirect=None, status=None)) after connection broken by 'ProxyError('Cannot connect to proxy.', NewConnectionError(': Failed to establish a new connection: [Errno 111] 拒绝连接',))': /simple/pip/
WARNING: Retrying (Retry(total=2, connect=None, read=None, redirect=None, status=None)) after connection broken by 'ProxyError('Cannot connect to proxy.', NewConnectionError(': Failed to establish a new connection: [Errno 111] 拒绝连接',))': /simple/pip/
WARNING: Retrying (Retry(total=1, connect=None, read=None, redirect=None, status=None)) after connection broken by 'ProxyError('Cannot connect to proxy.', NewConnectionError(': Failed to establish a new connection: [Errno 111] 拒绝连接',))': /simple/pip/
WARNING: Retrying (Retry(total=0, connect=None, read=None, redirect=None, status=None)) after connection broken by 'ProxyError('Cannot connect to proxy.', NewConnectionError(': Failed to establish a new connection: [Errno 111] 拒绝连接',))': /simple/pip/

尝试过修改不同的源，没用：
txs@txs-PC:~$ python3 -m pip install --upgrade pip -i http://pypi.douban.com/simple/ --trusted-host pypi.douban.com
Looking in indexes: http://pypi.douban.com/simple/
WARNING: Retrying (Retry(total=4, connect=None, read=None, redirect=None, status=None)) after connection broken by 'ProxyError('Cannot connect to proxy.', NewConnectionError(': Failed to establish a new connection: [Errno 111] 拒绝连接',))': http://pypi.douban.com/simple/pip/
WARNING: Retrying (Retry(total=3, connect=None, read=None, redirect=None, status=None)) after connection broken by 'ProxyError('Cannot connect to proxy.', NewConnectionError(': Failed to establish a new connection: [Errno 111] 拒绝连接',))': http://pypi.douban.com/simple/pip/
WARNING: Retrying (Retry(total=2, connect=None, read=None, redirect=None, status=None)) after connection broken by 'ProxyError('Cannot connect to proxy.', NewConnectionError(': Failed to establish a new connection: [Errno 111] 拒绝连接',))': http://pypi.douban.com/simple/pip/
WARNING: Retrying (Retry(total=1, connect=None, read=None, redirect=None, status=None)) after connection broken by 'ProxyError('Cannot connect to proxy.', NewConnectionError(': Failed to establish a new connection: [Errno 111] 拒绝连接',))': http://pypi.douban.com/simple/pip/
WARNING: Retrying (Retry(total=0, connect=None, read=None, redirect=None, status=None)) after connection broken by 'ProxyError('Cannot connect to proxy.', NewConnectionError(': Failed to establish a new connection: [Errno 111] 拒绝连接',))': http://pypi.douban.com/simple/pip/

请问这是什么原因呢？应该怎么解决？