在升级当前virtualenv版本的时候，报错：ERROR: Cannot uninstall 'virtualenv'. It is a distutils installed project and thus we cannot accurately determine which files belong to it which would lead to only a partial uninstall.
升级的时候执行的命令是：pip install --upgrade virtualenv
改成执行下面的命令就不会报错了：pip install --upgrade virtualenv --ignore-installed distlib