yo hicesudo pip install -U nltk como lo sugiere la documentación de nltk. Sin embargo, obtengo el siguiente resultado:

Collecting nltk
  Downloading nltk-3.0.5.tar.gz (1.0MB)
    100% |████████████████████████████████| 1.0MB 516kB/s 
Collecting six>=1.9.0 (from nltk)
  Downloading six-1.9.0-py2.py3-none-any.whl
Installing collected packages: six, nltk
  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/basecommand.py", line 211, in main
    status = self.run(options, args)
  File "/Library/Python/2.7/site-packages/pip/commands/install.py", line 311, in run
    root=options.root_path,
  File "/Library/Python/2.7/site-packages/pip/req/req_set.py", line 640, in install
    requirement.uninstall(auto_confirm=True)
  File "/Library/Python/2.7/site-packages/pip/req/req_install.py", line 716, in uninstall
    paths_to_remove.remove(auto_confirm)
  File "/Library/Python/2.7/site-packages/pip/req/req_uninstall.py", line 125, in remove
    renames(path, new_path)
  File "/Library/Python/2.7/site-packages/pip/utils/__init__.py", line 315, 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: '/tmp/pip-7dp3on-uninstall/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/six-1.4.1-py2.7.egg-info'

Básicamente, no puede actualizar seis de 1.4.1 a 1.9.0. Traté de hacerlo por separado pero obtuve el mismo error.

Menciono El Capitán porque algunas personas enfrentan problemas para otra instalación de Python en El Capitán y me pregunto si esto se debe a eso.

¿Alguien sabe cómo solucionar este problema?