workonコマンドで仮想環境に入るときのエラー

#workonコマンドで仮想環境に入りたい

workonコマンドでvirtualenvwarapperで作った仮想環境に入りたい。


```
$ workon myvenv(仮想環境名)
```
これで入れるはずが以下のエラーが出る。

```
File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/runpy.py", line 162, in _run_module_as_main
    "__main__", fname, loader, pkg_name)
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/runpy.py", line 72, in _run_code
    exec code in run_globals
  File "/Library/Python/2.7/site-packages/virtualenvwrapper/hook_loader.py", line 16, in <module>
    from stevedore import ExtensionManager
ImportError: No module named stevedore
Traceback (most recent call last):
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/runpy.py", line 162, in _run_module_as_main
    "__main__", fname, loader, pkg_name)
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/runpy.py", line 72, in _run_code
    exec code in run_globals
  File "/Library/Python/2.7/site-packages/virtualenvwrapper/hook_loader.py", line 16, in <module>
    from stevedore import ExtensionManager
ImportError: No module named stevedore
```
仮想環境には入れるけどエラーが出る。
どうやらstevedoreというモジュールが無いらしい。

作業環境:
macOS Sierra バージョン10.12.5
Python 2.7.10
virtualenv 15.1.0
virtualenvwrapper 4.7.2

#対応策
stevedoreをインストールする。

```
$ sudo pip install stevedore
```
またエラーが出る。

```
Collecting stevedore
  Downloading stevedore-1.25.0-py2.py3-none-any.whl
Requirement already satisfied: pbr!=2.1.0,>=2.0.0 in /Library/Python/2.7/site-packages (from stevedore)
Collecting six>=1.9.0 (from stevedore)
  Downloading six-1.10.0-py2.py3-none-any.whl
Installing collected packages: six, stevedore
  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/7t/313m_dg1067bhtr2llqd0q9cz8c4fj/T/pip-KG9_ld-uninstall/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/six-1.4.1-py2.7.egg-info'
```

調べるてみるとsixが既に入ってることが原因らしい。
以下のコマンドでインストールし直してみる。

```
$ sudo pip install stevedore --upgrade --ignore-installed six
```

これでエラーが消えました。

#参考文献
http://qiita.com/kawashinji/items/98fd8a917a17c3476091
https://github.com/pypa/pip/issues/3165