Use the correct __builtin__/builtins module in Python 2/3.

[matthiaskramm]

There is no "builtins" in Python 2.

[gvanrossum]

Strangely enough that doesn't appear to work with mypy! Mind if I change it as follows?

# workaround for mypy#2010
if sys.version_info < (3,):
    import __builtin__ as builtins
else:
    import builtins
_str = builtins.str