尝试使用pip安装任何软件包时出现此错误。我有两个pip实例，一个使用Python 2.7，另一个使用Python 3。

    Could not import setuptools which is required to install from a source distribution.
Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/pip/req/req_install.py", line 375, in setup_py
    import setuptools  # noqa
  File "/usr/local/lib/python2.7/dist-packages/setuptools/__init__.py", line 11, in <module>
    from setuptools.extern.six.moves import filterfalse, map
  File "/usr/local/lib/python2.7/dist-packages/setuptools/extern/__init__.py", line 1, in <module>
    from pkg_resources.extern import VendorImporter
ImportError: No module named extern

即使当我尝试安装“外部”模块时，也会出现此错误。同样在使用Python本身进行安装时，例如python setup.py install。

提前致谢。