python3 raise ImportError('This package should not be accessible on Python 3.

当电脑上安装了Python2 和 python3时，在python3下执行help()报如下错误：

raise ImportError('This package should not be accessible on Python 3. '
ImportError: This package should not be accessible on Python 3. 
Either you are trying to run from the python-future src folder or your installation of python-future is corrupted

这是因为环境变量中包含PYTHONPATH=/usr/local/lib/python2.7/site-packages，而这条不适用于python3。
在shell下执行

unset PYTHONPATH

即可解决

参考文献：
https://stackoverflow.com/questions/42214414/this-package-should-not-be-accessible-on-python-3-when-running-python3