pip安装依赖six-1.4.1失败解决方案

$sudo pip install matplotlib --upgrade

在安装matplotlib包的时候，遇到如下报错：

Installing collected packages: six,python-dateutil,pyparsing,numpy,pytz,backports.functools-lru-cache,cycler,subprocess32,matplotlib
Found existing installation: six 1.4.1
DEPRECATION: Uninstalling a distutils installed project (six) has been deprecated and will be removed in a future version. This is due to the fact that uninstalling a distutils project will only partially uninstall the project.
Uninstalling six-1.4.1:
Exception:
Traceback (most recent call last):
File "/Library/Python/2.7/site-packages/pip-9.0.1-py2.7.egg/pip/basecommand.py",line 215,in main
status = self.run(options,args)
File "/Library/Python/2.7/site-packages/pip-9.0.1-py2.7.egg/pip/commands/install.py",line 342,in run
prefix=options.prefix_path,
File "/Library/Python/2.7/site-packages/pip-9.0.1-py2.7.egg/pip/req/req_set.py",line 778,in install
requirement.uninstall(auto_confirm=True)
File "/Library/Python/2.7/site-packages/pip-9.0.1-py2.7.egg/pip/req/req_install.py",line 754,in uninstall
paths_to_remove.remove(auto_confirm)
File "/Library/Python/2.7/site-packages/pip-9.0.1-py2.7.egg/pip/req/req_uninstall.py",line 115,in remove
renames(path,new_path)
File "/Library/Python/2.7/site-packages/pip-9.0.1-py2.7.egg/pip/utils/__init__.py",line 267,in renames
shutil.move(old,new)
File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/shutil.py",line 302,in move
copy2(src,real_dst)
File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/shutil.py",line 131,in copy2
copystat(src,dst)
File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/shutil.py",line 103,in copystat
os.chflags(dst,st.st_flags)

OSError: [Errno 1] Operation not permitted: '/var/folders/c1/k8c_wsgj3n5b9zpr5mb284sw0000gn/T/pip-k1xxj6-uninstall/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/six-1.4.1-py2.7.egg-info'

错误提示表明，six 1.4.1已经存在，并且将来会被废弃，unintall的时候又因为权限的问题(six因安全问题，被apple设置为即使sudo也不能执行)而无法卸载，可以修改命令，跳过six的重装：

$sudo pip install matplotlib --upgrade --ignore-installed six