pip3 错误 - '_NamespacePath' 对象没有属性 'sort'

我尝试通过 pip3 安装软件包，但出现此错误.我运行的每个 pip/pip3 命令都会给我这个错误 -

I tried to install a package through pip3, and I got this error. Every pip/pip3 command that I run gives me this error-

alexg@hitbox:~$ pip3 -V
Traceback (most recent call last):
  File "/usr/local/bin/pip3", line 7, in <module>
    from pip import main
  File "/home/alexg/.local/lib/python3.5/site-packages/pip/__init__.py", line 26, in <module>
    from pip.utils import get_installed_distributions, get_prog
  File "/home/alexg/.local/lib/python3.5/site-packages/pip/utils/__init__.py", line 27, in <module>
    from pip._vendor import pkg_resources
  File "/home/alexg/.local/lib/python3.5/site-packages/pip/_vendor/pkg_resources/__init__.py", line 3018, in <module>
    @_call_aside
  File "/home/alexg/.local/lib/python3.5/site-packages/pip/_vendor/pkg_resources/__init__.py", line 3004, in _call_aside
    f(*args, **kwargs)
  File "/home/alexg/.local/lib/python3.5/site-packages/pip/_vendor/pkg_resources/__init__.py", line 3046, in _initialize_master_working_set
    dist.activate(replace=False)
  File "/home/alexg/.local/lib/python3.5/site-packages/pip/_vendor/pkg_resources/__init__.py", line 2578, in activate
    declare_namespace(pkg)
  File "/home/alexg/.local/lib/python3.5/site-packages/pip/_vendor/pkg_resources/__init__.py", line 2152, in declare_namespace
    _handle_ns(packageName, path_item)
  File "/home/alexg/.local/lib/python3.5/site-packages/pip/_vendor/pkg_resources/__init__.py", line 2092, in _handle_ns
    _rebuild_mod_path(path, packageName, module)
  File "/home/alexg/.local/lib/python3.5/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'

我尝试根据一些答案运行 pip3 install --upgrade pip setuptools，但每个 pip 命令都给我错误.我被卡住了，什么也做不了.

I have tried running pip3 install --upgrade pip setuptools according to some answers, but every pip command gives me the error. I'm stuck and can't do anything.

可能是什么原因造成的，如何解决?

What could have caused it, and how can this be fixed?

我在使用 Python 3.5.2

I'm on Ubuntu 16.04 with Python 3.5.2