pip3 installed with 3.6.3 crashes on startup. 
pip3 installed with 3.5.4 works. 
Appears on OS X 10.13.1

platt@MacBook-Pro.local:/Users/platt $ pip3
Traceback (most recent call last):
  File "/Library/Frameworks/Python.framework/Versions/3.6/bin/pip3", line 7, in <module>
    from pip import main
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/pip/__init__.py", line 26, in <module>
    from pip.utils import get_installed_distributions, get_prog
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/pip/utils/__init__.py", line 27, in <module>
    from pip._vendor import pkg_resources
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/pip/_vendor/pkg_resources/__init__.py", line 3018, in <module>
    @_call_aside
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/pip/_vendor/pkg_resources/__init__.py", line 3004, in _call_aside
    f(*args, **kwargs)
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/pip/_vendor/pkg_resources/__init__.py", line 3046, in _initialize_master_working_set
    dist.activate(replace=False)
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/pip/_vendor/pkg_resources/__init__.py", line 2578, in activate
    declare_namespace(pkg)
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/pip/_vendor/pkg_resources/__init__.py", line 2152, in declare_namespace
    _handle_ns(packageName, path_item)
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/pip/_vendor/pkg_resources/__init__.py", line 2092, in _handle_ns
    _rebuild_mod_path(path, packageName, module)
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/pip/_vendor/pkg_resources/__init__.py", line 2121, in _rebuild_mod_path
    orig_path.sort(key=position_in_sys_path)
AttributeError: '_NamespacePath' object has no attribute 'sort'

Thank you for your report. This is a problem with setuptools: https://github.com/pypa/setuptools/issues/885 I suggest upgrading both setuptools and pip:

    $ pip install -U setuptools pip

platt@MacBook-Pro.local:/Users/platt $ pip install -U setuptools pip
Collecting setuptools
  Downloading setuptools-36.7.2-py2.py3-none-any.whl (482kB)
    100% |████████████████████████████████| 491kB 2.6MB/s 
Requirement already up-to-date: pip in ./Library/Python/2.7/lib/python/site-packages
Installing collected packages: setuptools
  Found existing installation: setuptools 18.5
    Uninstalling setuptools-18.5:
Exception:
Traceback (most recent call last):
  File "/Users/platt/Library/Python/2.7/lib/python/site-packages/pip/basecommand.py", line 215, in main
    status = self.run(options, args)
  File "/Users/platt/Library/Python/2.7/lib/python/site-packages/pip/commands/install.py", line 342, in run
    prefix=options.prefix_path,
  File "/Users/platt/Library/Python/2.7/lib/python/site-packages/pip/req/req_set.py", line 778, in install
    requirement.uninstall(auto_confirm=True)
  File "/Users/platt/Library/Python/2.7/lib/python/site-packages/pip/req/req_install.py", line 754, in uninstall
    paths_to_remove.remove(auto_confirm)
  File "/Users/platt/Library/Python/2.7/lib/python/site-packages/pip/req/req_uninstall.py", line 115, in remove
    renames(path, new_path)
  File "/Users/platt/Library/Python/2.7/lib/python/site-packages/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 299, in move
    copytree(src, real_dst, symlinks=True)
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/shutil.py", line 208, in copytree
    raise Error, errors
Error: [('/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/_markerlib/markers.pyc', '/var/folders/68/lk7bsfq51hq3406jk8v83z880000gn/T/pip-hvP8_p-uninstall/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/_markerlib/markers.pyc', "[Errno 1] Operation not permitted: '/var/folders/68/lk7bsfq51hq3406jk8v83z880000gn/T/pip-hvP8_p-uninstall/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/_markerlib/markers.pyc'"), ('/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/_markerlib/__init__.py', '/var/folders/68/lk7bsfq51hq3406jk8v83z880000gn/T/pip-hvP8_p-uninstall/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/_markerlib/__init__.py', "[Errno 1] Operation not permitted: '/var/folders/68/lk7bsfq51hq3406jk8v83z880000gn/T/pip-hvP8_p-uninstall/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/_markerlib/__init__.py'"), ('/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/_markerlib/markers.py', '/var/folders/68/lk7bsfq51hq3406jk8v83z880000gn/T/pip-hvP8_p-uninstall/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/_markerlib/markers.py', "[Errno 1] Operation not permitted: '/var/folders/68/lk7bsfq51hq3406jk8v83z880000gn/T/pip-hvP8_p-uninstall/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/_markerlib/markers.py'"), ('/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/_markerlib/__init__.pyc', '/var/folders/68/lk7bsfq51hq3406jk8v83z880000gn/T/pip-hvP8_p-uninstall/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/_markerlib/__init__.pyc', "[Errno 1] Operation not permitted: '/var/folders/68/lk7bsfq51hq3406jk8v83z880000gn/T/pip-hvP8_p-uninstall/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/_markerlib/__init__.pyc'"), ('/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/_markerlib', '/var/folders/68/lk7bsfq51hq3406jk8v83z880000gn/T/pip-hvP8_p-uninstall/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/_markerlib', "[Errno 1] Operation not permitted: '/var/folders/68/lk7bsfq51hq3406jk8v83z880000gn/T/pip-hvP8_p-uninstall/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/_markerlib'")]

1) Don't reopen a closed issue.
2) Please use python-list or StackOverflow for usage questions. You need to replace 'pip' with 'pip3' in your case. If you get a similar error with 'pip3', try 'sudo pip3 install -U ...' or 'pip3 install --user -U ...'.