Python AttributeError: '_NamespacePath' object has no attribute 'sort'

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问题：
在使用源码安装的方式版本回退一个三方包后，使用 pip 命令时，均会出现以下报错

Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/pip/_vendor/__init__.py", line 33, in vendored
    __import__(vendored_name, globals(), locals(), level=0)
ImportError: No module named 'pip._vendor.pkg_resources'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/bin/pip3", line 9, in 
    from pip import main
  File "/usr/lib/python3/dist-packages/pip/__init__.py", line 13, in 
    from pip.exceptions import InstallationError, CommandError, PipError
  File "/usr/lib/python3/dist-packages/pip/exceptions.py", line 6, in 
    from pip._vendor.six import iteritems
  File "/usr/lib/python3/dist-packages/pip/_vendor/__init__.py", line 75, in 
    vendored("pkg_resources")
  File "/usr/lib/python3/dist-packages/pip/_vendor/__init__.py", line 36, in vendored
    __import__(modulename, globals(), locals(), level=0)
  File "", line 969, in _find_and_load
  File "", line 958, in _find_and_load_unlocked
  File "", line 664, in _load_unlocked
  File "", line 634, in _load_backward_compatible
  File "/usr/share/python-wheels/pkg_resources-0.0.0-py2.py3-none-any.whl/pkg_resources/__init__.py", line 2927, in 
  File "/usr/share/python-wheels/pkg_resources-0.0.0-py2.py3-none-any.whl/pkg_resources/__init__.py", line 2913, in _call_aside
  File "/usr/share/python-wheels/pkg_resources-0.0.0-py2.py3-none-any.whl/pkg_resources/__init__.py", line 2952, in _initialize_master_working_set
  File "/usr/share/python-wheels/pkg_resources-0.0.0-py2.py3-none-any.whl/pkg_resources/__init__.py", line 956, in subscribe
  File "/usr/share/python-wheels/pkg_resources-0.0.0-py2.py3-none-any.whl/pkg_resources/__init__.py", line 2952, in 
  File "/usr/share/python-wheels/pkg_resources-0.0.0-py2.py3-none-any.whl/pkg_resources/__init__.py", line 2515, in activate
  File "/usr/share/python-wheels/pkg_resources-0.0.0-py2.py3-none-any.whl/pkg_resources/__init__.py", line 2097, in declare_namespace
  File "/usr/share/python-wheels/pkg_resources-0.0.0-py2.py3-none-any.whl/pkg_resources/__init__.py", line 2047, in _handle_ns
  File "/usr/share/python-wheels/pkg_resources-0.0.0-py2.py3-none-any.whl/pkg_resources/__init__.py", line 2066, in _rebuild_mod_path
AttributeError: '_NamespacePath' object has no attribute 'sort'

解决方案：
这意味着依赖关系混乱了，尝试了 pip3 install --upgrade setuptools 等一些方法，未能处理这个问题。
最后我通过重新安装 pip 解决了这个问题。

curl https://bootstrap.pypa.io/get-pip.py -o get-pip.py
sudo python3 get-pip.py

相关讨论：
如果你按照以上方法仍没能解决，可查看以下相关讨论从中寻找其他可能的解决方案
https://stackoverflow.com/questions/47955397/pip3-error-namespacepath-object-has-no-attribute-sort