尝试progressbar在OSX El Capitan上安装python软件包时，出现以下错误：

Collecting progressbar
  Using cached progressbar-2.3.tar.gz
    Complete output from command python setup.py egg_info:
    Traceback (most recent call last):
      File "<string>", line 20, in <module>
      File "/private/var/folders/9s/b1jly7yd0jq639plvkzdvn4m0000gn/T/pip-build-xhfxywsw/progressbar/setup.py", line 5, in <module>
        import progressbar
      File "/private/var/folders/9s/b1jly7yd0jq639plvkzdvn4m0000gn/T/pip-build-xhfxywsw/progressbar/progressbar/__init__.py", line 59, in <module>
        from progressbar.widgets import *
      File "/private/var/folders/9s/b1jly7yd0jq639plvkzdvn4m0000gn/T/pip-build-xhfxywsw/progressbar/progressbar/widgets.py", line 121, in <module>
        class FileTransferSpeed(Widget):
      File "/Users/<USERNAME>/anaconda/lib/python3.4/abc.py", line 133, in __new__
        cls = super().__new__(mcls, name, bases, namespace)
    ValueError: 'format' in __slots__ conflicts with class variable

----------------------------------------
    Command "python setup.py egg_info" failed with error code 1 in /private/var/folders/9s/b1jly7yd0jq639plvkzdvn4m0000gn/T/pip-build-xhfxywsw/progressbar

更新以下内容无法解决我的问题

pip install --upgrade setuptools
pip install --upgrade Distribute

我该如何解决？