关于python 2.7：升级到ubuntu-16.10后，Pip不起作用

Pip does not work after upgrade to ubuntu-16.10

与pip一起运行命令会产生以下错误。即使命令 pip -V 也会产生以下错误。我了解到错误是由于安装工具版本31.0.0造成的，应该低于28.0.0。但我的安装工具版本是26.1.1，它仍然给出相同的错误。

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Traceback (most recent call last):
File"/usr/local/bin/pip", line 7, in
from pip import main
File"/usr/local/lib/python3.5/dist-packages/pip/__init__.py", line 26, in
from pip.utils import get_installed_distributions, get_prog
File"/usr/local/lib/python3.5/dist-packages/pip/utils/__init__.py", line 27, in
from pip._vendor import pkg_resources
File"/usr/local/lib/python3.5/dist-packages/pip/_vendor/pkg_resources/__init__.py", line 3018, in
@_call_aside
File"/usr/local/lib/python3.5/dist-packages/pip/_vendor/pkg_resources/__init__.py", line 3004, in _call_aside
f(*args, **kwargs)
File"/usr/local/lib/python3.5/dist-packages/pip/_vendor/pkg_resources/__init__.py", line 3046, in _initialize_master_working_set
dist.activate(replace=False)
File"/usr/local/lib/python3.5/dist-packages/pip/_vendor/pkg_resources/__init__.py", line 2578, in activate
declare_namespace(pkg)
File"/usr/local/lib/python3.5/dist-packages/pip/_vendor/pkg_resources/__init__.py", line 2152, in declare_namespace
_handle_ns(packageName, path_item)
File"/usr/local/lib/python3.5/dist-packages/pip/_vendor/pkg_resources/__init__.py", line 2092, in _handle_ns
_rebuild_mod_path(path, packageName, module)
File"/usr/local/lib/python3.5/dist-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'