from builtins import * introduces suble bugs

[g4b1nagy]

Consider the following code, running on Python 2.7.6:

>>> from builtins import *
>>> l = [1,2,3]
>>> type(l) is list
False

Edit: this also happens if you use from builtins import str:

>>> from builtins import str
>>> s = 'foo'
>>> type(str) is str
False

I would suggest adding a warning regarding the type() function when using builtins and providing isinstance() as an alternative.

[aaronchall]

Since isinstance is the general guidance for all Python type-checking, I don't think a special warning is warranted here.