pip的版本报错，以及WARNING: Retrying (Retry(total=4, connect=None, read=None, redirect=None, status=None))

使用pip时出现警告：
WARNING: There was an error checking the latest version of pip.
可以进行版本升级，用以下两个语句之一更新版本：

python -m pip install --upgrade pip
pip install --upgrade pip
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若还是不行，或者出现报错

WARNING: Retrying (Retry(total=4, connect=None, read=None, redirect=None, status=None)) after connection broken by 'ProxyError('Cannot connect to proxy.', OSError(0, 'Error'))': /simple/pip/
WARNING: Retrying (Retry(total=3, connect=None, read=None, redirect=None, status=None)) after connection broken by 'ProxyError('Cannot connect to proxy.', OSError(0, 'Error'))': /simple/pip/
WARNING: Retrying (Retry(total=2, connect=None, read=None, redirect=None, status=None)) after connection broken by 'ProxyError('Cannot connect to proxy.', OSError(0, 'Error'))': /simple/pip/
WARNING: Retrying (Retry(total=1, connect=None, read=None, redirect=None, status=None)) after connection broken by 'ProxyError('Cannot connect to proxy.', OSError(0, 'Error'))': /simple/pip/
WARNING: Retrying (Retry(total=0, connect=None, read=None, redirect=None, status=None)) after connection broken by 'ProxyError('Cannot connect to proxy.', OSError(0, 'Error'))': /simple/pip/
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考虑可能是挂着代理，把代理关掉看一下
或者降低pip版本，将pip版本降低的语句为：

python -m pip install pip==20.2.2
python -m pip install pip==20.2.2 -i http://pypi.douban.com/simple --trusted-host pypi.douban.com
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